WASHINGTON (Reuters) - Republican John McCain opened a narrow lead on rival Mitt Romney ahead of a critical Florida primary after picking up the endorsement of Gov. Charlie Crist, according to a Reuters/C-SPAN/Zogby poll released on Monday.

McCain gained 3 points overnight after winning the Florida governor’s backing on Saturday. The shift broke a tie at 30 percent and pushed McCain, an Arizona senator, into a 33 percent to 30 percent lead on Romney before Tuesday’s primary.

The margin of error in the poll is 3.4 percentage points.

The backing of Crist particularly helped McCain with conservatives. He wiped out a 6-point lead for Romney, a former Massachusetts governor, among voters who describe themselves as conservatives.

“You have to say the Charlie Crist endorsement clearly had an impact,” said pollster John Zogby. “McCain went from losing big among conservatives to a tie.”

Zogby said the endorsement gave McCain more credibility among hard-line Republicans and older voters in Florida ahead of the potentially critical showdown.

“One day doesn’t make a trend, but to gain 3 points in a day could be significant,” Zogby said.

The two candidates split last week’s three nominating contests as Republicans battle for the right to represent the party in November’s presidential election. McCain won in South Carolina and Romney won in Michigan and Nevada.

The winner in Florida will gain valuable momentum heading into the February 5 “Super Tuesday” voting, when 21 states will have Republican nominating contests in a sprawling coast-to-coast battle.

McCain, a former Vietnam prisoner of war, has stressed his foreign policy experience and his leadership on Iraq, including his backing of President George W. Bush’s strategy to raise troop levels there.

THE ECONOMY

Romney, a wealthy former venture capitalist, has countered by emphasizing his economic credentials and criticizing McCain’s understanding of the economy.

The poll found the economy is the top issue in Florida among likely voters, at 40 percent. The war on terrorism ranked second at 14 percent, with the war in Iraq in third at 12 percent.

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Despite a win in Iowa by former Arkansas Gov. Mike Huckabee and former New York Mayor Rudy Giuliani’s long lead in national polls last year, neither candidate has been able to gain traction in Florida as McCain and Romney leave them behind.

Giuliani, who pulled out of early voting states to concentrate on Florida, was at 14 percent. Huckabee was at 11 percent, down 3 points overnight, and Texas Rep. Ron Paul was at 2 percent.

About 8 percent of Florida voters are still undecided about their choice.

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Democrats also will hold a Florida primary, but a dispute with the national party over the contest’s date cost the state its delegates to the national convention and led presidential contenders to pledge they would not campaign there.

Hillary Clinton, a New York senator who was crushed in South Carolina on Saturday by Illinois Sen. Barack Obama, said she would go to Florida to greet supporters after voting ends -- technically honoring the pledge.

The rolling poll of 818 likely Republican voters was taken Friday through Sunday. It will continue for one more day.

In a rolling poll, the most recent day’s results are added while the oldest day’s results are dropped in order to track changing momentum.

(Editing by John O’Callaghan)