For all his momentum, Mitt Romney can't seem to shake Rick Santorum. A new NBC/Marist poll shows Romney with a 7 point lead going into Tuesday's Wisconsin primary:

In the upcoming Wisconsin primary, billed as perhaps the final opportunity to change the trajectory of the Republican presidential contest, frontrunner Mitt Romney leads Rick Santorum by seven percentage points, according to a new NBC News/Marist poll. But should he capture the nomination, Romney would start out as the underdog against President Barack Obama, whom Romney trails by double digits.

In Wisconsin's April 3 Republican contest, the former Massachusetts governor gets support from 40 percent of likely primary voters, including those who are undecided yet leaning toward a particular candidate. Former Pennsylvania Sen. Rick Santorum gets 33 percent, Texas Rep. Ron Paul gets 11 percent,  and former House Speaker Newt Gingrich gets 8 percent. Seven percent of respondents are undecided.

The poll - conducted March 26-27 - is consistent with the findings of a recent Marquette Law School survey, which found Romney leading Santorum by eight points. The Wisconsin race follows a familiar pattern: Romney holds the advantage over Santorum among liberal and moderate Republicans (43 percent to 24 percent), conservatives (42 percent to 33 percent), non-Tea Party supporters (42 percent to 31 percent), and those who earn $75,000 or more annually (47 percent to 32 percent)

Santorum wins among very conservative voters, tea party members, and evangelicals.

Romney picked up the endorsement of Rep. Paul Ryan yesterday, but these high profile announcements never seem to make much difference. What matters is who shows up at the polls, and as long as Santorum excites his supporters enough to get them out on election day, he will be competitive.