Republicans in the District of Columbia, Maryland and Wisconsin will cast their ballots today for the GOP nomination. Polls show the closest race should be Wisconisn where Rick Santorum is within 10 points of Romney in most polls.

Report from USAToday:

WAUKESHA, Wis. – Primary voters here will either help solidify Mitt Romney’s grip on the Republican presidential nomination or give Rick Santorum another reason to continue his campaign.

With 42 delegates to the Republican National Convention at stake in Tuesday’s primary, Wisconsin can’t get Romney to the 1,144 he needs to secure the nomination. It can, however, add to Romney’s growing “perception of inevitability,” says John McAdams, a political scientist at Marquette University.

Citing state polls that show Romney leading, McAdams says, “It will be a major shock if Romney doesn’t win.” If Santorum pulls off an upset, McAdams says, “he still remains an underdog.”

Santorum said Sunday on NBC’s Meet the Press that the Wisconsin primary is not “do or die” for him. Maryland, with 37 delegates, and Washington, D.C., with 19, also vote Tuesday.

Wisconsin has not backed a Republican presidential candidate in a general election since 1984 when Ronald Reagan was on the ballot, but Democrats’ margin of victory in 2000 and 2004 was a few tenths of a percentage point. President Obama beat Republican Sen. John McCain here in the 2008 election, 56% to 42%.

The Republican establishment has begun coalescing around Mitt Romney and the calls for Santorum and Gingrich to end their respective campaigns is growing louder. The month of April could be a Waterloo for Santorum who will need to keep winning delegates at every contest to change the flow of the race. At this point, he’s a long shot to keep positively improving his trajectory.