(Reuters) - Mitt Romney's lead over Newt Gingrich has grown to 15 percentage points in Florida, a Reuters/Ipsos poll released on Monday found on the day before the state's Republican primary.

Support for Romney, a former Massachusetts governor and private equity executive, edged up to 43 percent among likely voters in Florida's January 31 primary, according to the online tracking poll. It had been at 42 percent on Sunday.

But backing for Gingrich, a former speaker of the House of Representatives, slipped to 28 percent, down from 30 percent support a day earlier, 32 percent on Saturday and 33 percent on Friday.

Romney is holding a good solid position and Gingrich continues to lose support ... which results in a bigger margin for Romney, said Chris Jackson, research director for Ipsos public affairs.

The gap between Romney and Gingrich was 14 percentage points when voters were asked who they would support in a hypothetical head-to-head race among the two contenders for the Republican nomination to oppose President Barack Obama in the November general election.

Romney was at 56 percent to Gingrich's 42 percent among likely voters asked that question. The gap had been 11 points a day earlier.

The well-funded and well-organized Romney had two strong debate performances last week and has consolidated his lead in Florida over Gingrich while hitting his rival with a blizzard of attack advertisements, funded by his campaign and by outside groups supporting his presidential aspirations.

SANTORUM AND PAUL FADE

Two other contenders for the Republican nomination, former Pennsylvania Sen. Rick Santorum and Texas Rep. Ron Paul, were well behind the two leaders. Santorum's support was at 12 percent, down from 16 percent in Sunday's results. Paul was backed by 5 percent of survey respondents, down from 6 percent on Sunday.

Paul has not been campaigning in Florida.

We're starting to see the impact of Santorum suspending his campaigning in Florida, Jackson said.

Santorum said on Sunday he was canceling his campaign events in Florida after his 3-year-old daughter was admitted to a hospital in Philadelphia.

Florida allows early voting at polling stations and by mail, and more than one-third - 35 percent - of respondents to the online poll said they had already cast their ballots. Romney's lead was particularly strong among that group, at 48 percent to 26 percent, 22-point margin.

Barring some sort of miracle for Newt Gingrich, this one's starting to look a little bit more wrapped up, and we can start thinking about Nevada Jackson said.

After Florida's primary on Tuesday, Nevada's caucuses on Saturday are the next contest in the state-by-state process of choosing a Republican nominee to run against President Barack Obama.

So far, the contests have been divided among the candidates. Santorum won Iowa's caucuses on January 3, Romney won the New Hampshire primary on January 10 and Gingrich took South Carolina's primary on January 21.

A big victory in Florida, by far the largest of the early-voting states, would be a giant step in Romney's push toward securing the nomination.

Statistical margins of error are not applicable to online surveys, but this poll of 658 likely voters in the Florida primary has a credibility interval of plus or minus 4.4 percentage points for registered voters.

Monday's Reuters/Ipsos survey is the last of four daily tracking polls that were released ahead of Tuesday's Florida primary.

(Editing by Eric Walsh)