The Republican presidential candidates opened an intensive week of campaigning in wide-open Iowa on Tuesday with the embattled Newt Gingrich casting rival Mitt Romney as an establishment defender of big government and accusing Romney’s supporters of lying about his record.

Gingrich, who has fallen after holding the lead a month ago, went to work trying to limit the damage done by the millions of dollars’ worth of negative ads aimed at him in recent weeks and counter the growing impression that Romney is suddenly well positioned to win in Iowa and consolidate a grip on the nomination.

The prospect of a Romney victory in Iowa has alarmed virtually everyone else in the Republican field. Iowa has long been seen as a difficult state for the former Massachusetts governor because of the heavily conservative leanings of many activists here. Were he to win next week and couple that with an expected victory in New Hampshire, Romney would put himself in a commanding position to win the nomination.

Romney, who began his day in New Hampshire, offered no predictions about Iowa but sounded a confident note about his overall prospects. “I think I’m going to get the nomination if we do our job right,” he said before flying to Iowa.

Romney arrived in Davenport later in the day and was met by an overflow crowd as he began a three-day bus tour of Iowa with an opportunity to win the caucuses, despite a far more limited effort than he put forth before losing four years ago. He focused his rhetoric on President Obama, saying he had promised but failed to put Americans back to work. “Mr. President, you have now had your moment,” he said. “We have seen the results. And now, Mr. President, this is our time.”

The campaign sprang to life Tuesday after a Christmas-weekend hiatus and suddenly featured all the traditional elements of past battles here — television commercials clogging local news­casts, candidates’ buses crisscrossing the state and volunteers engaged in a last dash to nail down support ahead of next week’s first-in-the-nation caucuses.

Both Romney and Gingrich were keeping an eye on Rep. Ron Paul (Tex.), the libertarian conservative whose organizational prowess has put him in the thick of the battle. Paul was due to resume his campaigning on Wednesday.

On the western side of the state, three of the most conservative candidates — former senator Rick Santorum (Pa.), Rep. Michele Bachmann (Minn.) and Texas Gov. Rick Perry — battled to coalesce the religious and social conservatives who have played a significant role in some previous campaigns but who have been fragmented in this year’s race. All three believe that, with a strong enough showing, they can survive the caucuses and keep their candidacies alive for later contests.

This year’s campaign is unusual because the scrambled race here has given all the candidates at least some reason to hope they will be able to keep going beyond Iowa. But the real focus over the next week will be on Romney, Gingrich and Paul.

Gingrich has been heavily outspent by Romney, Paul and Perry, but he will begin airing a new ad on Wednesday, according to campaign officials. In addition, an independent super PAC backing his candidacy will spend more than $1 million nationally over the next week, most of it in Iowa, according to Rick Tyler, the spokesman for the PAC and a former top aide to Gingrich.

Gingrich acknowledged last week that the attacks have hurt him. Among the charges leveled against him by his rivals: that he is a Washington insider, that he is not a true conservative and that he supported a Democratic effort to pass restrictions on carbon emissions.

Gingrich is also under fire for saying several years ago that he liked Romney’s plan to reform health care in Massachusetts, though now, like most Republicans, he criticizes its provision for an individual insurance mandate. He is also facing revelations that his first wife did not want a divorce, contrary to his own account of the dissolution of the marriage.

On Tuesday, Gingrich defended his long history of supporting conservative candidates and causes in American politics, from Barry Goldwater to Ronald Reagan to Jack Kemp’s tax-cutting theory of supply-side economics. He also repeatedly criticized Romney.

“I don’t want to be invidious about Governor Romney, who I think is a very competent manager and a very smart man,” Gingrich said. “But to have someone who is a Massachusetts moderate, who said he did not want to go back to the Reagan-Bush years, who voted as a Democrat for Paul Tsongas in ’92, who campaigned to the left of Teddy Kennedy, who as recently as running for governor said, ‘I’m really sort of a moderate pragmatic’ — to have him run a commercial that questions my conservatism? I mean, I’ve been a conservative all my life.”

Gingrich acknowledged that his appearance in a video with former House speaker Nancy Pelosi about climate change was “the dumbest thing I’ve done in the last four years.” But he accused his critics of lying about his support for the cap-and-trade bill championed by Pelosi and Obama. He said he had testified against the bill and helped block it in the Senate after it passed the House.

“If somebody wants to run an ad and say, ‘There are moments when Newt Gingrich shows a total lack of judgment, and this was one of them,’ I’ll laugh with the rest of them,” he said. “And I don’t mind an opponent who takes a clean shot and says, ‘You know, I gotcha.’ But they then go from there, and frankly they plain lie. I don’t know any other word for it.”

Gingrich attacked Paul in even sharper terms. During an interview on CNN, he said the Texas congressman’s foreign policy positions as well as his recent claims that he knew nothing of the racially charged and anti-Semitic content of his newsletter disqualify him for the presidency. “The people of the United States are not going to accept somebody who thinks it’s irrelevant if Iran gets a nuclear weapon. I think that’s a national security threat of the first order.”

Responding to the controversy over writings in newsletters that Paul’s organization published in the 1990s, Gingrich said, “There will come a moment when people don’t take him seriously.” Gingrich said that if Paul were the nominee, he would not support him.

Criticism of Paul has begun to escalate as polls have shown him to be a threat to win the caucuses. Coming this close to the voting, the attacks could be a critical setback to his candidacy, but his supporters are believed to be among the most passionate and devoted of any of the candidates’.

Staff writer Sandhya Somashekhar in New Hampshire contributed to this report.