Wisconsin could effectively end the Republican race

For these reasons, in fact, I am suspicious that the Rasmussen Reports poll in Wisconsin is a modest outlier. But Mr. Santorum is clearly no longer the favorite there, and if he were to lose the state by anything like the 13-point margin that Rasmussen Reports projects, it could potentially put an end to the campaign.

Wisconsin uses a relatively aggressive delegate allocation method, as do Maryland and the District of Columbia, making it possible that Mr. Romney could win almost all of the 96 delegates at stake on Apr. 3 if he had a good night. That could put Mr. Romney ahead by close to 400 delegates overall even if he lost Louisiana, a commanding advantage that would be close to mathematically impossible for Mr. Santorum to overcome.

Finally, Mr. Santorum is losing ground to Mr. Romney in the Gallup national tracking poll, which now shows Mr. Romney with 40 percent of the vote for the first time. A tracking poll can fluctuate quite a bit, but it is at least possible that some Republicans who like Mr. Santorum but want to avoid a brokered convention are migrating to Mr. Romney.