Mitt Romney won primaries in Wisconsin, Maryland and the District of Columbia on Tuesday, but Rick Santorum has vowed to continue his now hopeless campaign through at least the end of the month.

CNN:

The former Massachusetts governor is expected to take the majority of Wisconsin's 42 delegates and Maryland's 37 delegates, with some designated solely for the winner and the rest to be awarded proportionally. He will take all 16 delegates at stake in the District of Columbia.

Counting partial allocations for Wisconsin and Maryland, and full allocations for D.C., Romney has collected 648 delegates since the primary and caucuses began in January, according to CNN estimates. That's more than twice the 264 delegates Santorum is estimated to hold. Gingrich and Paul trailed well back.

With 93% of the vote reported in Wisconsin, Romney had 42% and Santorum, a former U.S. senator from Pennsylvania, had 38%. U.S. Rep. Ron Paul of Texas had 12%, and former House Speaker Newt Gingrich had 6%.

In Maryland, with 75% of the vote reported, Romney had 49%. Santorum had 29%, Gingrich had 11% and Paul had 10%.

In the District of Columbia, with 99% of the vote reported, Romney had 70%. Paul had 12% and Gingrich had 11%. Santorum was not on the D.C. ballot.

[...]

But Santorum told supporters in Mars, Pennsylvania, on Tuesday night that the race was essentially at halftime, with only roughly half the available delegates awarded.

"Pennsylvania and half the other people in this country have yet to be heard, and we're going to go out and campaign here and across this nation to make sure that their voices are heard in the next few months," Santorum said.

Why does Santorum persist despite not having any realistic shot at the nomination? Clearly, a big reason is he wants to keep faith with his supporters. And he wants to have a chance to run in his home state of Pennsylvania later this month where he may have designs for future political office. Santorum will be 54 next month which certainly is young enough for the candidate to dream about other opportunities in Pennsylvania.

But perhaps the biggest reason Santorum wants to stay in the race is to keep Mitt Romney honest and influence the GOP platform. He genuinely believes a strong conservative candidate will fare better against Obama in the fall and if that candidate can't be him, he would dearly love to shape the issues upon which Romney will have to run in November.

This presents Romney with something of a dilemma. Mitt would like to be able to ignore Santorum and begin developing his themes against Obama. But he risks experiencing some surprise losses that wouldn't slow his drive to the nomination, but would make him look weak even in his own party. So Romney will keep one eye on Santorum for the next few weeks while working in some attacks on President Obama. It's not an ideal situation for Romney, but it's the reality of what he is facing at the moment.