WASHINGTON - Mitt Romney's above-the-fray campaign style kept him atop the Republican presidential field for months, but it is raising concerns among his supporters now that Newt Gingrich has surged to challenge him.

Some Romney backers say their candidate must mix it up more aggressively, with Gingrich and with reporters, to prove he has the moxie to be the GOP challenger to President Obama. The former Massachusetts governor particularly caused concerns with his prickly responses in a recent Fox News interview, and he needs to show more toughness and willingness to field questions, party insiders say.

The message seemed to resonate with his campaign Tuesday. Romney said he would appear Dec. 18 on Fox News Sunday, his first national Sunday talk show in nearly two years. He also fielded questions from reporters covering his Arizona visit, his third "press availability" in four days.

The moves make sense to Rich Galen, a GOP strategist and former Gingrich aide who is neutral in the current race.

"The lack of engagement strategy has served Romney pretty well," Galen said. "Now I think they've got to alter course and get him out there more."

Numerous Romney supporters had expressed concern over reports of his dodging reporters and in-depth questioning.

"It remains a mystery why Mitt Romney has done relatively few interviews," Jennifer Rubin, a conservative blogger for the Washington Post who often praises Romney, wrote Monday.

The much-discussed Nov. 29 Fox interview, Rubin said, might have gone better "had it been one of dozens of TV interviews he'd given during the campaign. ... He's been the least interviewed candidate in the race."

In his 15-minute exchange with Fox News' Bret Baier, Romney bristled at questions about his changed views on abortion, global warming, immigration, and gay rights, all of which are widely discussed in political circles.

Romney acknowledged rejecting his pro-abortion-rights stand of the 1990s, though he did not explain why. Otherwise, he told Baier, "your list is just not accurate." Romney suggested the questions were inspired by "Democratic ads" that label him a serial flip-flopper.

On Tuesday, Romney hinted that he might slowly ramp up his criticism of Gingrich.

"I will not be quiet," he told Fox News' Neil Cavuto. Gingrich "is a friend, respected. But we have very different life experiences."

Romney said Gingrich had spent the last 40 years or so in Washington, "working as an insider." Romney says he would bring a more business-oriented, outside perspective.

Also Tuesday, Romney said he would not participate in a Dec. 27 debate being hosted by real estate mogul and TV personality Donald Trump. Among the GOP candidates, so far only Gingrich and former Pennsylvania Sen. Rick Santorum have said they will attend.

Romney also picked up the endorsement Tuesday of former Vice President Dan Quayle.