Today Pennsylvania, New York, Connecticut, Rhode Island, and Delaware hold their Republican primaries. Mitt Romney was poised to do well even when Rick Santorum was still actively campaigning, with Pennsylvania the only obviously competitive state. Without Santorum, Romney is well positioned for a clean sweep.

But it will still be interesting to see how big the remaining anti-Romney vote is. Newt Gingrich is trying to win Delaware, where even some Romney supporters claim the former speaker has some chance. In the absence of reliable polling data, it is hard to know whether this is a pipe dream. But Christine O’Donnell upset Mike Castle in the First State in 2010. Steve Forbes also briefly revived his flagging 1996 presidential campaign in Delaware. Forbes, the only candidate to campaign in the state that year, won unexpectedly and gave himself a boost going into the Arizona primary.

Santorum was a two-term senator from Pennsylvania, where he will still be on the ballot. The polls were starting to move against him even while he was still in the race, but states sometimes reward native sons. I noted this weekend that Howard Dean won Vermont in 2004 after suspending his campaign.

Ron Paul won 20 out of 24 delegates in Minnesota. He had finished second in the popular vote in the state’s caucus. There are reports of six other caucus states where Republicans are bracing for a possible delegate coup by the Paul forces, who remain active and relatively well funded.

I”m not predicting any huge upsets tonight, but we could learn how little or much work Romney still has to do to win over his party.