Mitt Romney previewed his “closing argument” to Republicans here Tuesday and said he will begin a more aggressive campaign as he tries to regain his lead in the presidential field with just four weeks until voting begins in Iowa.

Romney, who has been overtaken in polls by former House speaker Newt Gingrich (Ga.), promised a new wave of television advertisements and media appearances. Although he declined to participate in a Dec. 27 debate that will be moderated by real estate mogul Donald Trump, Romney plans to make his first Sunday talk show appearance in nearly two years on Dec. 19, on “Fox News Sunday.”

He told reporters in Arizona that Gingrich’s poll numbers do not faze him. “We’re just starting,” he said. “We’re just getting up with ads. We’re making our closing argument. You’ll see me campaigning aggressively. . . . This will probably take longer than a week or two to sort out. My expectation is this will be a campaign that’s going to go on for a while, and I expect to win it.”

Stumping in Arizona, where he picked up the endorsement of former vice president Dan Quayle, Romney emphasized a contrast with President Obama that his aides say will be one of his central themes heading into the Jan. 3 Iowa caucuses.

“Are we going to have a society of merit and opportunity or a nation instead of entitlement?” Romney said. “Are we going to have a government that is so strong that it manages our lives and manages our enterprises and manages our economy? Or are we going to have people so strong that they manage their own lives and build their own enterprises?”

Romney will double down on his core message about the economy, as well as talk about his temperament, to present a subtle contrast with Gingrich, a Romney adviser said.

Romney plans to spend more time campaigning in South Carolina, which holds the third contest, and his wife, Ann, will have her own events there to provide a reminder of the candidate’s family life, the adviser said. The campaign also plans to show more organizational muscle in states such as Florida, where Romney has a broad network of support.

Romney declined opportunities to attack Gingrich at a news conference here Tuesday.

But earlier, in an interview with Fox News, he said: “Just wait and see.” He added: “We’re going to make sure that the differences in our experience and perspective are well aired.” “You can be sure I will not be quiet. I am going to make sure my message is heard loud and clear.” About 150 people attendedthe event — held at the Hermosa Inn, a four-star boutique hotel in the affluent Phoenix suburb of Paradise Valley — and received valet parking. The campaign’s new theme song, “Born Free” by Kid Rock (who, like Romney, is a Detroit native) played as the attendees sipped iced tea and waited under the cool desert sun

Last week, Romney began a line of attack against Gingrich by labeling him a “career politician.” Gingrich pushed back by noting how many times Romney has run unsuccessfully for office, dating to his 1994 Senate bid. Asked by a reporter here how he would respond, Romney said: “I ran for office, but I didn’t win.”

“When I lost elections, I got to go back and be a businessman again and it allowed me to be involved in the economy and to understand at the grass-roots level why Americans are suffering and what I can do to get jobs again,” Romney said.

Romney and Quayle addressed them, facing the red rocks of Camelback Mountain, with the hotel’s cactus garden as their backdrop. Quayle praised Romney as “our best hope to take on President Obama.”

Staff writer Karen Tumulty in Washington contributed to this report.