MILWAUKEE, Wisc. (Reuters) - For Republican presidential candidate Rick Santorum, it is shaping up to be a cold and lonely spring.

Republican presidential candidate and former U.S. Senator Rick Santorum plays a friendly game of shuffle board at Title Town Brewery in Green Bay, Wisconsin March 24, 2012. REUTERS/Darren Hauck

Trailing Mitt Romney in the polls, the conservative former senator is pinning his slim hopes on surviving difficult primary votes in April that favor his rival, and then trying to recover in May when the calendar looks better for him.

“There were lots of times throughout the course of this campaign where, as I’ve said before, we were running a marathon breathing through a swizzle stick. And that may be the case again going through the month of April,” Santorum told Reuters in an interview on Saturday.

If Santorum can survive until May, he has a fair chance of winning states where his appeal to evangelical voters goes down well, such as North Carolina, West Virginia, Indiana, Kentucky and perhaps Texas, with its large haul of 155 delegates.

“Major states have not been heard from. And we think we can do well in a lot of those states particularly in May,” he said.

All the same, Santorum has a mountain to climb to accumulate the 1,144 delegates needed to win the nomination. Romney has an estimated 565 delegates, according to the Real Clear Politics website, followed by Santorum with 256.

Calls for Santorum to quit the race will increase, especially if he fails to win Tuesday’s Wisconsin primary, but he is likely to stay in until at least the vote in his home state of Pennsylvania on April 24.

“The amount of people that will call for him to step aside will grow over the next few weeks,” said Republican strategist Ron Bonjean. “As the party tries to unite around Romney, he’ll become more unpopular.”

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REAL PHONE CALLS

Romney leads opinion polls in Wisconsin and is using his huge financial advantage to launch attacks against Santorum.

Automatic “robocalls” to Wisconsin voters paid for by the Romney campaign paint Santorum as a supporter of big spending and labor unions during his time in the Senate.

Way behind Romney in the money stakes, Santorum’s campaign cannot compete directly with the robocalls. He is betting on a cheaper, more personal approach, with campaign workers making one call at a time.

“I don’t know how many people have come up to me and said they’ve gotten five, six, ten robocalls. You know we’re making volunteer calls. We’ve got real people on the phone asking people to vote for us. And I think that’s much more effective,” Santorum told Reuters.

On Sunday, Wisconsin Senator Ron Johnson followed the lead of the state’s most popular Republican, Congressman Paul Ryan, in endorsing Romney.

Santorum, who said on Saturday he would not be “unrealistic” if it became clear that he cannot win the nomination, may put up one final stand in Pennsylvania, even though he lost his Senate seat there by 18 points in 2006.

“I think Santorum feels he has nothing to lose, and he really doesn’t,” said Bonjean.

“So why not try to carry this on for another month to Pennsylvania and see how well he can do there? These losses he’s racking up don’t seem to matter to him. What else is he going to do?”

Pennsylvania is a large, moderately expensive state of the type where Romney does well once his campaign and “Restore Our Future” Super PAC group weigh in with TV ads.

Santorum is aware he has an uphill fight there and said moderate Republicans in the state will be working against him.

“They’re very active in the Romney campaign and ... would like nothing better than to change the face again of the Republican Party in Pennsylvania and I think they’re seeing this as a way to do that,” he said.