ORLANDO, Fla. — Mitt Romney is poised to win big in Tuesday’s Florida Republican primary, a victory that would move him an important step closer to the party’s presidential nomination.

About 2 million people are expected to cast ballots in the biggest primary of the 2012 campaign so far. Most polls suggest a double-digit Romney victory over his three rivals, and the former Massachusetts governor is likely to tout a strong showing as proof that he has broad appeal to a diverse electorate.

But the race won’t be over.

“A Romney victory will leave us where we thought it was several months ago, with Romney the likely nominee. But a lot of Republicans still are not reconciled to that idea,” said Larry Sabato, the director of the University of Virginia Center for Politics.

Newt Gingrich, a former speaker of the House of Representatives, had hoped to capitalize on the momentum from his South Carolina landslide over Romney on Jan. 21. The two began last week in a virtual Florida tie in most polls.

But the well-funded Romney camp unleashed an ad barrage questioning Gingrich’s ethics and work for mortgage titan Freddie Mac. And Romney attacked Gingrich sharply in two televised debates, while Gingrich’s performances failed to provide the boost that two South Carolina debates had given him there. Last, iconic figures from the GOP establishment — from former presidential nominees John McCain and Bob Dole to the late William F. Buckley’s National Review — pronounced Gingrich erratic and unfit for the presidency.

The attacks scored. Polls tell the tale: Romney holds a 12.5 percentage-point lead, according to an average of nine surveys taken from last Tuesday through Sunday — all after the first Florida debate — compiled by Real Clear Politics, a nonpartisan website. A Quinnipiac University survey conducted Friday through Sunday had Romney ahead among every subgroup of voters: conservatives, evangelicals, young, old, rich, poor and middle class.

Gingrich has vowed to stay in the race until the August convention. Texas Rep. Ron Paul made only a token effort in Florida, but he’s mounting a strong effort in Nevada, the site of the next caucus, on Saturday.

Former Pennsylvania Sen. Rick Santorum all but abandoned his Florida effort over the weekend. He’s expected to be in Nevada on Tuesday.

Romney alone has the organization and the money needed to blanket each competitive state with ads and workers. He raised $24 million in the fourth quarter of last year, far more than any of his rivals.

In Florida, Romney and a “super” political action committee that’s backing him have spent $15.9 million on TV ads so far, while Gingrich and his supporters have spent about $4 million, according to data compiled by NBC.

Because of the Republicans’ complex delegate-selection rules, no one is likely to amass the 1,144 delegates needed for nomination until at least April.

“It could be a long haul,” Sabato said. “You know the ending, but you don’t know how beaten up Romney will be.”

The victor in Tuesday’s winner-take-all primary will get 50 delegates. After Nevada on Saturday, the next contests will come in Colorado, Minnesota and Missouri: All three vote Feb. 7. Maine voters are encouraged to caucus from Saturday to Feb. 11.

By the end of February, only about 15 percent of the convention delegates will have been chosen. The biggest one-day delegate prize comes March 6, when 10 states will select a total of 437 delegates. The biggest batch that day will be 76 delegates in Georgia, Gingrich’s home state.

Romney and Gingrich resumed trying to beat each other up Monday as they crisscrossed Florida.

Romney spoke at a Jacksonville forklift-supply company, where he emphasized that Gingrich was a paid consultant for mortgage giant Freddie Mac as the housing foreclosure crisis deepened.

“That’s the real reason why Speaker Gingrich has had such a hard time,” Romney said. “If (people) want to see change in Washington, you can’t just select the same people to take different chairs.”

Romney mocked a campaign promise that Gingrich made on Florida’s Space Coast last week that, as president, he’d establish a permanent U.S. colony on the moon.

“The idea of the moon as the 51st state is not what would come to my mind as a campaign basis for here in Florida,” Romney said.

Gingrich, who traveled by plane Monday, addressed a small but enthusiastic crowd at a Tampa Jet Center hangar. His campaign had curtained off two-thirds of the hangar, but the crowd still didn’t fill the remaining third.

Gingrich pounded away at Romney as a liberal who’s beholden to Wall Street.

“He can bury me for a very short amount of time with four or five or six times as much money, most of it raised on Wall Street from the guys who got bailouts from the government,” he told ABC. “In the long run, the Republican Party is not going to nominate the founder of Romneycare, a liberal Republican who is pro-abortion, pro-gun control.”

Romney signed into law a Massachusetts health care plan that’s considered the model for the 2010 federal law, which he and Gingrich want to repeal. Romney denies that he’s pro-abortion or pro-gun control.

Preceding Gingrich at the rally were President Ronald Reagan’s son Michael and Herman Cain, who suspended his own presidential campaign early last month after sexual harassment allegations.

Michael Reagan touted Gingrich’s Reagan credentials. Romney’s campaign has pointed out that Ronald Reagan mentioned Gingrich only once in his diaries. Pat Buchanan, who was Reagan’s White House Communications director, said over the weekend that Reagan’s team had viewed Gingrich as a “political opportunist” who was “not trusted” and “played no role whatsoever” in shaping Reagan’s policies.

“The people who are saying he wasn’t there, weren’t there,” Michael Reagan said.

He had no role in his father’s presidency, however, and Buchanan did.