“I do believe I would’ve voted for Mitt Romney,” Ms. Durand said, explaining a change of heart as the race has developed, though she did not say she regretted her absentee ballot. “I like Rick Santorum’s consistency, his views on life, all of his views, but I think the strongest ticket offer to do the ultimate goal of beating President Obama would be Romney.”

Ms. Durand, 32, was at a vigil outside a clinic that provides abortion services on Milwaukee’s Lower East Side, 10 miles north of where Mr. Santorum visited on Saturday. He was the favored candidate of several people interviewed around the state, but others said they planned to vote for Mr. Romney because they thought he was more likely to win the general election.

The Republican presidential race has steadily narrowed to a two-man contest — for now, at least — between Mr. Romney and Mr. Santorum. Newt Gingrich and Representative Ron Paul of Texas finished well behind their rivals in Louisiana, failing to reach the threshold to win any delegates. Yet they pledged to stay in the race, which has complicated the path for Mr. Santorum and frustrated some of his supporters.

“There’s nothing wrong with Santorum and Romney staying in there,” said Tom Fredrich, 48, an undecided Republican voter who came to see Mr. Santorum on Sunday at the Ledgeview Bowling Lanes in Fond du Lac. He said that Mr. Gingrich and Mr. Paul should “just go away right now,” adding that he drew the line at a contested convention.

“A divided party cannot beat Obama,” Mr. Fredrich said. After listening to Mr. Santorum speak, Mr. Fredrich and his wife, Bridget, walked away as committed supporters, complete with a sign vowing to “join the fight.”

A friendly primary calendar in March, in which Mr. Santorum has carried seven states, gives way to more difficult terrain for him in April. He has struggled to expand his coalition of supporters beyond evangelical Christians and voters with lower incomes and less formal education. After Wisconsin, there is a three-week break, and the contest resumes in Pennsylvania, New York and a series of states with some of the most expensive advertising rates in the country.

The Romney campaign, after initially holding back in calling for its rival to leave the race, has intensified its criticism. Ryan Williams, a spokesman for Mr. Romney who was standing in the back of the Santorum event here, said Mr. Santorum sounded “less like a person who is committed to helping conservatives defeat Barack Obama and more like a spoiled child who wants to take his ball and go home.”