Each party's base has two inconvenient truths it doesn't want to hear. For Republicans, those truths concern immigration and the culture war. Most of today's illegal immigrant population is here to stay (along with their descendants) and will pay no significant price for getting here outside the legal channels. No presidential candidate can change those facts. On the issue that matters most to conservative Christians--abortion--the political phase of the culture war is over. The right lost --a pro-life initiative failed in South Dakota in 2006: If it can't win there, it can't win anywhere. Well, maybe Utah.

For Democrats, the relevant subjects are Iraq and federal spending. Discussions of the Iraq war in Democratic primaries have a bizarre quality: Both Hillary Clinton and Barack Obama speak as though the war is a lost cause. It isn't--unless one of them wins the election and pulls the plug, a scenario that Iran's proxies no doubt await eagerly. As for spending, the federal budget (and federal tax revenues) will leave no room for large, expensive, New Deal-style health and education programs. For the foreseeable future, domestic policymaking will have more to do with arranging incentives than with dispensing largesse: Think welfare reform, not Aid to Families with Dependent Children.

If Republicans fail to understand their unpleasant truths, they will lose in November, and lose badly. Democrats might win even if their heads remain in the sand: It's a Democratic year, as a comparison between the two parties' fundraising, turnout, and vote totals in the primaries to date suggests. But they will lose the chance to have the kind of public debate that shapes government policy--meaning, the kind that is based on truth, convenient and otherwise.

Consider the four issues in turn. The Republican base wants the country to reacquire control over its southern border, and wants to see the millions of illegal immigrants already here expelled or punished--because anything less rewards them for their violations. The first goal is both good policy and good politics. The second is a practical impossibility and a political disaster. No American government can afford to track down and expel, fine, or otherwise penalize 12 million of its residents: 17 times the number of convicted felons who enter prison each year (and today's imprisonment rate has shattered historical records). That much law enforcement is beyond government's capacity--a fact for which conservatives, of all people, should be thankful.

Not only will the illegals themselves remain, so will generations of their offspring: a large voting bloc that will be forever barred to the party that wanted to ship their parents and grandparents back to their Central American homes. If the penalties for illegally crossing the border are more than a pittance, immigrants will simply refuse to pay them and remain underground, and no future government will spend the money needed to catch and prosecute them. Given those circumstances, amnesty is less a policy choice than a statement of political reality: the rough equivalent of bankruptcy for a debtor who, without it, will never pay another creditor another dime. To put the point differently, the size of America's Latino population means that the nation's border control problem must be solved with that population's consent. As Donald Rumsfeld might put it, you do immigration reform with the immigrants you have.

If the crusade to enforce our current immigration laws against our current immigrant population was lost several million border crossings ago, the crusade to end abortion and reform the culture by means of electoral politics was lost several election cycles ago. In 1989, William Rehnquist's Supreme Court issued its decision in Webster v. Reproductive Health Services, hinting that Roe v. Wade's reversal was just around the corner. That fall, Virginians chose the nation's first elected black governor--not in spite of the fact that he was pro-choice, but because of it. Political insiders have long understood what many pro-life voters are loath to admit: In any national election in which abortion rights were squarely at issue, the pro-choice side would win, and win big.

It may be just as well: Even if culture warriors' political agenda were achievable, that agenda might prove counterproductive. Cultures are powerful and mysterious things; the idea that laws and politicians can direct their paths is, to say the least, lacking in empirical support. In the years immediately before Roe, abortion was a crime, and the number of abortions soared. Since that decision, abortion has been a constitutional right--yet, since 1980, the abortion rate has fallen by more than one third. The lesson is one conservatives should find easy to understand: Like modern economies, modern cultures resist centralized control. If pro-life evangelicals--of whom I'm one--wish to persuade our fellow citizens to protect unborn life, we must persuade them, not prosecute the ones who disagree.

Republicans may be slow to accept defeat, but Democrats seem to have trouble accepting victory. It is no longer possible to say with a straight face that the war in Iraq is as good as lost, or that the "surge" is a flop. David Petraeus has proved to be a 21st-century Matthew Ridgway: the general who took over American forces in Korea after the Chinese had taken Seoul and swept down the peninsula. Ridgway retook Seoul, pushed Chinese and North Korean forces back to the 38th parallel, and salvaged a partial victory from what had looked like certain defeat. Petraeus has done as much, in more difficult circumstances. Yet Obama and Clinton compete to see who condemned the war soonest and who can promise to withdraw American soldiers the fastest.

They're missing the point. The war can and should be won even if it shouldn't have been fought in the first place--because we're not in the first place; choices must be made from where one stands today, not some imaginary place of the speaker's choosing. And the promise of speedy withdrawal tells those who fight American soldiers: Hold on a little longer; those you fight will soon leave the field. A more destructive message can scarcely be imagined.

The message on spending is simpler: Whatever programs the Democratic primary electorate may want, the money to pay for them will be there. It isn't so. Thanks to the Bush administration's budgetary surge--and, even more, thanks to the still-unsolved entitlements problem--the next president will have less budgetary room for maneuver than the current one had when he entered office.

In terms of fiscal policy, the last eight years have been a replay of the 1980s, when deficits soared but the Reagan administration paid no political price for them. The Reagan years were followed by the deficit-hawkish 1990s, when voters rewarded budget-balancing more than either tax-cutting or profligate spending. Today's candidates should take note. Senators Clinton and Obama may understand the fiscal constraints under which the next administration will operate--but if their debates about health care and education are any indication, they aren't telling Democratic voters.

A common thread runs through these four issues. Shooting wars and culture wars, immigration and the federal budget--all are examples of the Mick Jagger principle of governance: You can't always get what you want. Politicians have a limited array of governance tools available to them, and they operate in a world of constrained choices. Voters no doubt wish that the consequences of past policy errors--the Iraq invasion for Democrats, the failure to police the nation's borders for Republicans--could be wiped away. They can't. One part of the electorate believes that providing the best health care to all Americans is a moral imperative. Another part believes that the hundreds of thousands of abortions each year in the United States amount to a holocaust. No matter how passionately held, the first belief cannot make top-flight, universal, affordable health care a reality. Nor can pro-lifers' commitment, by itself, eliminate abortion. Wishing doesn't make it so.

A sizable share of both parties' most passionate supporters seem not to understand that basic truth. "Yes, we can!"--so goes the shout that punctuates most Obama rallies. The audience may not realize it, but the spirit of those words is uncomfortably close to the spirit that led the current administration to fight a hard war with too few weapons and too few soldiers. Sometimes, leaders need to say: No, you can't. Problems aren't solved because voters, or the politicians they support, imagine the solutions.

Because these are Democratic-leaning times, Republicans have the most to gain from embracing this year's inconvenient truths--and may have a nearly ideal candidate to do the embracing. John McCain may be better positioned than anyone in either party to secure the southern border without alienating America's Latino population. He has a strong pro-life voting record, but has never been in the thick of the culture wars. On Iraq, McCain is prominently identified with Petraeus and the surge. Politically, he stands in much the same position today as Dwight Eisenhower in 1952: tough-minded and hard-nosed without being reckless--and, like Eisenhower with Korea, he bears none of the blame for the war's mishandling. On spending, McCain may be the country's leading proponent of fiscal discipline: Ross Perot without the lunacy. A McCain-led Republican party could become the party of deficit hawks--just when deficits are about to become the political liability they were in the 1990s.

The two Democrats seem less impressive on this score. Hillary Clinton and Barack Obama talk about border control the way children talk about eating their vegetables. As kids leave the table before the beans and carrots are gone, one suspects a Democratic administration might quit on border security before the borders are secured. Neither sounds much like a deficit hawk. And on the war--the real one--both have made statements that could make wise governance impossible if either reaches power. Political talk matters: It shapes voters' expectations and defines the political context in which decisions are made. Standing tough in Iraq may be impossible after voters have heard, again and again, that their new president is firmly committed to bailing out, as quickly as possible.

Sad to say, the candidate who most often tells unhappy truths may not turn out to be the candidate who wins the most votes. Elections are not always won by truth-tellers; deception sometimes carries the day. John F. Kennedy, whose presidency is often invoked these days, won a close national election by describing an imaginary gap between the Soviet Union's arsenal of missiles and our own. If something similar happens this year, if the next president wins by promising limitless spending with limited taxes or a costless retreat in Iraq, voters should not blame the winning candidate. In politics as in markets, customers rule; we usually get the leaders we want. The trick is to want the right leaders. We might start by asking who tells us the truth--even, or especially, when it hurts.

William J. Stuntz is the Henry J. Friendly professor at Harvard Law School.

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