DES MOINES, Iowa (AP) -- Displaying the confidence of a front-runner, former Massachusetts Gov. Mitt Romney swept into Iowa on Tuesday ahead of next week's caucuses, shrugged off criticism from his Republican rivals and unleashed an attack on President Barack Obama.

"Mr. President, you have now had your moment. We have seen the results. And now, Mr. President, it is our time," Romney said in excerpts of a speech released in advance by his campaign.

Romney spoke as his rivals vied in increasingly acerbic terms to emerge as his principal, conservative rival in the long march of primaries that will follow the caucuses. The Jan. 3 event is the official kick-off of the competition for delegates to next summer's Republican National Convention.

(In related news, Romney compares Newt Gingrich to Lucille Ball: video.)

The strongest rhetoric of the day came from former House Speaker Newt Gingrich, who said unequivocally 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, whose views verge on libertarianism, shows a "systematic avoidance of reality."

In a measure of the political stakes, the candidates and allied groups have spent more than $12 million on television commercials to air through caucus day next Tuesday. Romney, Texas Gov. Rick Perry and supporting groups 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.

"I've been a conservative all my life," said Gingrich. 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 Texas Rep. Ron Paul is Romney's likeliest threat in Iowa. He is due in the state on Wednesday.

A conservative with libertarian leanings, Paul commands strong allegiance from his own 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.

And Paul was alone among the GOP contenders in a recent debate in saying the United States should not consider preemptive 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.

According to one tally of television advertising in the state, the 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.

Whatever the outcome, 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 Steve Peoples in New Hampshire contributed to this report.

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