WASHINGTON -- Mitt Romney has been steadily building a lead in pledged delegates, but Tuesday's Illinois primary may be where he regains the perception of momentum in the Republican nomination contest. Four surveys released in the last week all show the former Massachusetts governor leading, while the two most recent ones have him gaining support, with his margins over former Sen. Rick Santorum hitting double digits.

The two most recent telephone surveys in Illinois were conducted over the weekend. A live interviewer survey fielded by the American Research Group (ARG) showed Romney leading Santorum by 14 percentage points (44 to 30 percent), while an automated, recorded voice conducted by the Democratic Party-affiliated firm Public Policy Polling (PPP) gives Romney a 15-point advantage (45 to 30 percent).

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Three prior surveys showed Romney leading by narrower margins. An automated Rasmussen Reports poll conducted on March 15 showed Romney leading Santorum by nine percentage points (41 to 32 percent), while an automated survey conducted on March 14 by WeAskAmerica and Fox Chicago News found Romney leading by six points (37 to 31 percent). A live interviewer telephone survey conducted from March 7 to 9 by the Chicago Tribune and television station WGN showed a slightly closer, four-point Romney margin (35 to 31 percent).

The five polls all found former House Speaker Newt Gingrich and Rep. Ron Paul (R-Texas) running far behind the two front-runners. Gingrich received the support of between 12 and 14 percent of likely Republican primary voters, while Ron Paul won between 7 and 10 percent.

The Rasmussen Reports survey included a hypothetical question offering a choice between just Romney and Santorum, and it showed Romney's lead narrowing to just four percentage points (48 to 44 percent). "Given that 32% of voters [say they] could still change their mind before Tuesday," the Rasmussen analysis concludes, "this represents a potential opportunity for Santorum." The report also points out that just over half of Gingrich's supporters are especially uncertain, with 55 percent saying they could shift their support.

While a late collapse of support for Gingrich in Illinois would likely help Santorum, the surveys show no evidence of any erosion of support for the former Speaker over the past 10 days.

Polls in the earlier 2012 primaries and caucuses have sometimes understated support for Santorum, particularly in last week's primaries in Alabama and Mississippi. However, those polling errors have either occurred in very low-turnout caucuses or in states with very large numbers of evangelical Christians among the Republican electorate.

But Illinois polls do not seem as likely to underestimate Santorum's support. In the 2008 primary, just a third (33 percent) of Republican primary voters identified as white, evangelical Christians on the network exit poll. In last week's primaries, however, exit polls shows 75 percent of the primary voters in Alabama and 80 percent in Mississippi identified as born-again or evangelical Christian.

As the National Journal's Ron Brownstein has observed, the demographic patterns of support that have emerged in the Republican primaries and caucuses so far -- by ideology, income, education and religious affiliation -- have been remarkably stable. Romney does consistently best among college-educated, upper-income and suburban moderates while the non-Romney candidates (either Santorum or Gingrich, depending on the contest) have run strongest with lower-income Republicans, strong conservatives and evangelical Christians.

The same stable demographic patterns have enabled RealClearPolitics' Sean Trende to create a statistical model based only on county-level demographic data that accurately predicted Romney's performance in last week's Alabama, Mississippi and Hawaii primaries.

As of March 13, Trende's model predicted Romney would win 47 percent of the Illinois vote cast for Romney, Santorum and Gingrich combined, assuming that the past demographic patterns persist. The model would translate to roughly 42 to 44 percent of the total vote, if Ron Paul receives the vote share forecast by the polls.

A Romney win in Illinois in the mid-to-upper 40 percent range forecast by the PPP poll and Trende's model could translate to a significant gain in delegates for Romney, since 54 of Illinois' 69 convention delegates are chosen by direct election within each of the state's 18 congressional districts. If a candidate wins a majority of the vote in a congressional district, that candidate will likely gain all of the delegates elected within that district. Not surprisingly, the PPP poll showed Romney leading Santorum by wide margins in the suburban areas of the state (50 to 29 percent) as well as in the urban centers (46 to 23 percent). But it also showed Romney and Santorum nearly even in rural areas, where Romney had a two point (38 to 36 percent) edge.

If Romney is able to challenge Santorum even in the rural congressional districts, he could score a big win in the battle for momentum and delegates on Tuesday.

This article has been updated to include the ARG poll released on Monday morning and to correct the field dates on the Rasmussen poll and the characterization of Sean Trende's forecast for the Illinois primary. The original version incorrectly said the Rasmussen poll was conducted "over the weekend" and characterized Sean Trende's Illinois forecast for Romney as a percentage of the total vote.

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