For Republicans, poll numbers have never looked better prior to a midterm election than they do today. “You’ve got to pinch yourself every time you look at the data,” says pollster Neil Newhouse. A Republican victory “could be bigger than anyone thinks.”

But how predictive are the numbers? Are they reliable enough, assuming they don’t change significantly in the 2.5 months before the November 2 election, that we should expect a massive Republican sweep?

The answer may be found by comparing today’s numbers on the two poll questions that matter most – presidential job approval and the generic ballot – with those in the landslides of 1982, 1994, and 2006. Hint: the comparisons should encourage Republicans.

For the comparisons, I’ve used only Gallup Poll numbers, both for this week and for the three previous elections.

As of August 18, President Obama’s job performance was approved by 41 percent and disapproved by 52 percent, the worst showing of his presidency. Republicans had a 50 percent to 43 percent advantage on the generic ballot, which asks respondents whether they intend to vote for a Republican or Democrat for the House of Representatives. Gallup’s August polls involve registered voters. The polls closer to an election were narrowed to likely voters, a group that tends to be slightly more Republican than all registered voters.

The general rule has been that when a president’s rating dips into the mid-to-low 40s (or lower), his party is likely to lose a substantial number of House seats in a midterm election and some in the Senate as well. And this trend is reinforced when the opposition party is ahead on the generic ballot.

In 2006, President Bush’s approval was 42 percent in mid-August and fell to 37 percent in the final poll before election day. In August, Democrats led Republicans, 47 percent to 45 percent, on which party’s candidate was preferred. The final, pre-election poll gave Democrats an advantage of 51 percent to 44 percent. The election result: Democrats won 31 House and 6 Senate seats, capturing control of both bodies.

In 1994, President Clinton’s approval rose from 40 percent in August to 46 percent in early November. But the Republican lead on the generic question also rose, from 47 percent to 44 percent in August to 53.5 percent to 46.5 percent in November. The result: Republicans gained 54 seats in the House, 8 in the Senate, winning a majority in both chambers.

In 1982, President Reagan’s performance in office was approved by 41 percent in August and 42 percent in late October. Democrats had a whopping 54 percent to 36 percent advantage in August and a 55 percent to 45 percent lead in October. The result: Democrats won 27 House seats but lost one in the Senate.

Why didn’t Democrats win more seats in 1982, given the strength reflected in poll numbers? With 242 House seats before the election, they were near their high water mark, post-Depression, in the size of their majority. By boosting it to 269 seats, Democrats erased most of their losses in 1980.

Based on poll comparisons – similarities, actually – Republicans ought to pick up dozens of House seats and at least a handful of Senate seats this fall. In fact, it would be difficult for them not to do this well.

For the most part, the poll numbers in 2006, 1994, and 1982 changed little between August and November. That’s the usual pattern. The numbers aren’t likely to change in 2010 either. The public’s assessment of the economy – 2-to-1 negative in Gallup – is usually settled this close to an election. And neither Obama nor congressional Democrats are moving away from the liberal policies that have proved unpopular among a majority of voters.

Besides, there are other measures that favor Republicans. In 2002 and 2006, there were 3 million more Democratic primary voters than Republican voters, according to political analyst Rhodes Cook. In 2010, turnout has flipped. Kristin Davison, who works for Karl Rove, has tallied the primary vote so far this year: 15,116,218 Republicans to 11,720,080 Democrats.

This reflects the so-called enthusiasm gap with Republicans more excited about the election and more likely to vote. Another measure: By a lopsided margin, voters feel the country is moving in the wrong direction.

Pollster David Winston believes Republicans have better prospects in 2010 than they did in 1994. “This administration has opened the door wide for Republicans,” he says. “It’s even more open than it was in 1994.”

But Winston says Republicans “have the challenge of what people remember from 2006,” when Republicans were voted out of office in Congress. And they must answer what Winston calls the “why us?” question. They need to tell voters “what you’re going to get if we have a Republican majority.”

This doesn’t mean they have to adopt a specific proposal on every issue, according to Winston. But since voters are focused on jobs and the economy, Republicans must “lay out the Republican plan to grow the economy and create jobs.” He expects Republicans to do this next month.

Neil Newhouse, with his experience in polling, refuses to get too excited about 2010. The most he’ll say is Republicans “have a shot” at winning the House. He adds: “No pollster ever lost his job by being conservative.”

UPDATE: Karl Rove calculated recently that with Obama’s approval at 41 percent, Democrats will lose 35 House seats. His basis for this projection: “A look at the August job approval of Presidents since 1966 alongside the result of their first midterm.” However, “the historical record also shows that August job approval and midterm results are not perfectly correlated,” Rove wrote. Presidents Carter and Reagan lost fewer seats than their August ratings indicated; Presidents Clinton and LBJ lost more.

Rove has also pointed to the political model of Nate Silver, a Democratic analyst. It projects that a tie in the overall House vote nationally would lead to a Republican pickup of 30 seats. But if Republicans lead by 10 points in the popular vote, as they do today when the chronic underestimate of the Republican vote is accounted for, they’d win close to 70 seats. They need 39 to win back the House.

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