Des Moines, Iowa - Projecting the confidence of a front-runner, former Massachusetts Gov. Mitt Romney swept into Iowa on Tuesday ahead of next week's presidential caucuses, looked past his Republican rivals and accused President Barack Obama of "misguided policies and weak leadership" in the White House.

"Mr. President, you have now had your moment. We have seen the results. And now, Mr. President, it is our time," Romney said. Aides added that, by design, he spoke not far from where Obama campaigned four years ago this week en route to a caucus victory that set him on the road to the presidency.

Romney unleashed his attack on Obama as other Republican contenders vied in increasingly acerbic terms to emerge as his principal, conservative rival in the long march of primaries to follow the Jan. 3 caucuses.

Former House Speaker Newt Gingrich, slipping in recent polls, said he would not vote for Rep. Ron Paul if the Texan is the party's opponent against Obama next fall. In an interview on CNN, Gingrich said Paul holds "views totally outside the mainstream of virtually every decent American," adding that Paul believes it doesn't matter if the Iranians have a nuclear weapon.

There was no immediate response from Paul, who has run television ads critical of Gingrich.

In a measure of the political stakes, the candidates and allied groups have spent more than $12 million on commercials to air through caucus day next Tuesday. Romney, Texas Gov. Rick Perry and groups supporting the two men account for nearly half the total, according to one estimate.

Most of Romney's rivals preceded him into the state during the day at the end of a holiday lull, seeking support in caucuses that are likely to dispatch one or more of them to a hasty campaign exit.

"My idea of gun control? Use both hands," said Perry, setting out on a bus tour in hopes of resurrecting his once-promising candidacy. He also toughened his position on abortion, saying it should be prohibited in all cases.

"I've been a conservative all my life," said Gingrich at a campaign stop. He called Romney a "Massachusetts moderate ... who campaigned to the left of Teddy Kennedy."

In Dubuque, the first stop of a bus tour through the state, Gingrich said his own economic proposal for an optional flat-tax as well as the elimination of all capital gains taxes was a more pro-growth approach than Romney's prescription.

In a radio interview, former Pennsylvania Sen. Rick Santorum said Romney had "sort of gotten a pass"' when he said in a recent debate he had done all he could as Massachusetts governor to block same-sex marriages in the state.

Rep. Michele Bachmann of Minnesota had a bus of her own, and saw herself as the rightful Romney alternative.

"I am the only consistent conservative in the race and the only candidate with the proven leadership and experience to create more American jobs and repair our economy," she wrote in an email seeking donations for her underfunded candidacy.

Bachmann, Perry and Gingrich have all spent time atop the Iowa public opinion polls in recent months, either alone or alongside Romney, only to fall back.

Recent soundings suggest Paul is Romney's likeliest threat in Iowa. The Texan is due in the state on Wednesday.

A conservative with libertarian leanings, Paul commands strong allegiance from his supporters but appears to have little potential to expand his appeal and emerge as a serious challenger for the nomination.

Unlike his rivals and most Republican voters, he says the federal government should have no authority to ban abortion.

He was alone among the GOP contenders in a recent debate in saying the United States should not consider pre-emptive military action to prevent Iran from acquiring nuclear weapons, an issue of significant importance to Israel. He warned against jumping the gun, adding, "That's how we got into that useless war in Iraq."

Romney, making his second run for the nomination, has relied on a well-funded and disciplined organization, generally strong debate performances and deep-pocketed allies to keep his balance as others have risen to challenge him and fallen back.

Four years ago, he invested heavily in the state and wound up second, a finish that began the unraveling of his candidacy.

He plans to campaign across the state at least through Friday.

According to one tally of television advertising in the state, the former Massachusetts governor and a super PAC run by supporters have spent $3.7 million combined on ads through Jan. 3

The total was exceeded only by a combined $5.5 million for Perry and a super PAC set up by his supporters.

As if previewing the themes of a general election campaign, he said that in his campaign travels, "I've heard stories of The Great Obama Recession, of families getting by on less, of planned-for retirement replaced by jobs at minimum wage."

He said that "Gone is the "hope and change' candidate of Davenport. ... Instead the campaigner-in-chief divides Americans, engages in class warfare and resorts to distortion and demagoguery."

Whatever the outcome of the caucuses, there was a recognition that for some, Iowa might simultaneously be the first and last test of the campaign.

"If I finish dead last way behind the pack I'm going to pack up and go home," Santorum said in a radio interview on WHO in Des Moines. "But I don't think that's going to happen," he added instantly.

Santorum, more than any of the others, has campaigned in Iowa the old fashioned way by doggedly visiting all 99 counties and holding hundreds of town hall meetings.

In Mason City, on a final swing through the state, he, like the others, urged potential caucus-goers to look past the appeal of conservative pretenders.

"The siren song of "this person can win' has been the mantra of a lot of the candidates," he said. "Vote for me because I can win."

In the state where caucuses propelled Obama toward the White House in 2008, the president's campaign organization pointed toward Election Day next Nov. 6.

With offices in eight Iowa cities, officials said Obama's re-election campaign has placed hundreds of thousands of phone calls since April to potential supporters.

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Associated Press writers Chuck Babington in Des Moines, Tom Beaumont in Mason City, Philip Elliott in Council Bluffs, Shannon McCaffrey in Dubuque and Kasie Hunt in Davenport, Iowa, and Steve Peoples in New Hampshire contributed to this report.