Mitt Romney won a clean sweep Tuesday night, with victories in the District of Columbia, Maryland, and Wisconsin. It is the latter state I want to focus on, as it was the most important of the bunch (from a political standpoint), and caps off an interesting back-and-forth between Romney and Rick Santorum in the Great Lakes region.

Last week I commented that Santorum might have an angle on victory in Wisconsin because the rural vote could be larger there than in the previous Midwestern battles. That did not pan out. In fact, the rural vote was substantially reduced last night relative to the 2008 battle between Mike Huckabee and John McCain in Wisconsin.

Perhaps the surge in the rural vote in 2008 was due to the appeal of Huckabee among rural Republicans, or maybe moderates in greater Milwaukee were attracted to the Obama candidacy, leaving only rural voters to participate. Regardless, with Wisconsin looking more like Michigan and Ohio this time around, Santorum stood little chance of victory there.

So, the big question: did Romney develop any signs of momentum last night? In other words, was there any evidence that the signs of his “inevitability” shift some would-be voters his way?

The answer is, yes. We saw Romney make some strides among Santorum’s key groups – the rural vote, the socioeconomically downscale vote, and the very conservative. Let’s take each in turn.

First, the rural vote. Romney has been losing this bloc by wide margins in the Midwest. He lost them again Tuesday, but he made noticeable improvements.

Notice that this is Romney’s best showing among rural voters in the Midwest to date, and substantially improved relative to Ohio.

Second, the socioeconomically downscale, which we have been measuring by looking at income and college degrees.

Romney again won the upscale vote, but he also won the downscale, carrying voters who make less than $50,000 per year and who do not have a college degree. This sweep is the first in the Great Lakes region for the former Massachusetts governor.

Third, ideology.

This is the most substantial improvement we have seen for Romney. Though he had been losing the very conservative vote by double-digits in the Midwest for months, last night he tied these voters with Santorum.

So what does all of this mean?

Well, Romney’s steps are tentative – after all, Santorum still did reasonably well with his key groups and overall won a not-too-shabby 38 percent of the vote, which has roughly been his range in these contests. Nevertheless, we did see some real movement toward Romney among Santorum’s key groups last night.

So is this race over?

Not quite yet. While there is no doubt that Santorum cannot hope to match Romney in terms of votes or delegates by the time the race is over, he still does have one path toward the nomination, although it is very narrow. A win in Pennsylvania could in theory jumpstart his campaign, undo the tentative movement we saw toward Romney in Wisconsin tonight, and give Santorum a boost heading into the final leg of the contest. If that were to pan out, Santorum could argue – plausibly – that his late stage momentum gives him an equally strong moral claim to the nomination as Romney has. This is similar to what Ronald Reagan claimed against Gerald Ford in 1976 and Ted Kennedy against Jimmy Carter in 1980.

However, this is much easier said than done. His biggest problem is the unfriendly territory left. Of the 20 contests still remaining, I would count Romney the clear favorite in 11 of them, and Santorum in just 6. Plus, Romney still has huge delegate hauls waiting for him California and New York. That means that it is not simply enough for Santorum to hold his core coalition together, he also has to start making inroads into Romney’s base vote – the upscale, the urban/suburban, the moderate and somewhat conservative. He has not begun to do that yet, and the end is quickly approaching.

A second problem: It is unclear to me that Santorum is a slam-dunk for winning the Keystone State. The polls have shown a tightening race, the core demographics favor Romney, and Santorum is not beloved there, having lost 16 percent of Republicans and 20 percent of conservatives to Bob Casey Jr. in 2006. Factor in Romney’s considerable financial advantage and the growing air of inevitability around his candidacy, and I would count this race a toss-up at this point.

Third, and perhaps most significant of all is the sustained calls from the grand poo-bahs of the GOP for Santorum to exit. These chants have already begun, to little effect so far, but Santorum’s concern has to be that voters in his coalition (or those in Romney’s coalition who might be persuaded) start to heed that message, and coalesce around Romney. We saw something similar happen to Rick Perry after his defeat in Iowa – he pledged to go on to South Carolina, and fought on for a little while, but his dreadful poll position suggested that the writing was on the wall.

So, in the final analysis, I would conclude that the results in Wisconsin suggest that the race is not quite over, but it is looking more likely than ever before that Mitt Romney will be the Republican nominee in the 2012 general election.

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