DES MOINES, Iowa • The volatile struggle for votes in Tuesday's Iowa Republican presidential caucuses featured two different fights Thursday.

One involved a top tier of candidates with clear momentum in these final days before the nation's first caucuses. Mitt Romney and Rick Santorum were beaming with optimism Thursday about their prospects, while no-nonsense Texas U.S. Rep. Ron Paul delivered his somber message to big, devoted audiences.

Romney, a former Massachusetts governor, was confident enough to beef up his Iowa schedule, including a stay in Des Moines on Tuesday night to await results. He, Paul and Santorum, a former Pennsylvania U.S. senator and the favorite of Iowa's influential evangelical community, all hosted jam-packed, enthusiastic rallies Thursday.

The other squabble featured three candidates who are struggling to stay viable: former House of Representatives Speaker Newt Gingrich, who has fallen sharply since his brief time as a front-runner earlier this month, Minnesota U.S. Rep. Michele Bachmann and Texas Gov. Rick Perry.

Perry unleashed a radio ad featuring an announcer, "Wink Tax-and-Spend," who hosts a "Wheel of Washington" game show and compares Santorum with a pig because he supported federal budget earmarks for local projects. Perry has been fighting for the evangelical vote, which appears from recent polls to be tipping toward Santorum.

Bachmann had to convince voters that her campaign is still alive after Iowa state Sen. Kent Sorenson, who had chaired her state campaign, abruptly defected to the Paul camp Wednesday.

Gingrich continued to push for votes on a last-ditch bus tour around the state. He defended himself against rivals' claims that he'd lobbied in support of the 2003 Medicare prescription drug law, according to news reports from Sioux City. Conservatives have long been wary of the law, saying it helped balloon the federal deficit. Gingrich said he didn't lobby but simply supported the measure.

Despite the media attention to the latest polls, Tuesday's outcome remains difficult to predict, since many voters are still candidate-shopping.

Lyn Watson, an Ames homemaker, voted for Romney in the 2008 caucus but is considering Paul this time. "He feels so strongly about his views," she said, "and he has a good base of people following him."

Still, she won't rule out Romney: "Maybe he could work better in Washington," she said.

About 100 of Paul's followers jammed a small meeting room at the historic Hotel Pattee in Perry, where they heard their candidate reiterate his pet themes of slashing government and reining in U.S. intervention abroad. He also denounced sanctions against Iran's nuclear program, saying the Iranian people could interpret the measures as "an act of war."

At the eastern end of the state, Santorum basked in a late-campaign surge that's moved him into third place in some polls.

Romney traveled to Mason City, where he beamed with optimism and spent his time blasting President Barack Obama rather than fellow Republicans.

"I feel like breaking into '76 Trombones,'""'' Romney said, referring to the signature song of "The Music Man," the musical written by Mason City native Meredith Willson.