Republican U.S. presidential candidate Marco Rubio talks with Fox News Channel anchor Megyn Kelly during an interview at a campaign rally in Miami Thomson Reuters WASHINGTON (Reuters) - Republican presidential candidate Marco Rubio voiced regret on Wednesday about making personal attacks against front-runner Donald Trump, a strategy that has failed to slow the New York billionaire's momentum.

Rubio, Trump, Ohio Governor John Kasich and U.S. Senator Ted Cruz of Texas, the four remaining Republican presidential candidates, participated in a round of Fox News-sponsored town hall events a day before they meet for another debate on Thursday in Miami.

Rubio, whose campaign is fighting for survival after a series of disappointing performances in nominating contests for the Nov. 8 election, has drawn fire for a line of attack in recent weeks in which he got personal with Trump.

He called him a con artist and said among other things that he had small hands, a charge that Trump took to mean as questioning the size of his manhood.

Until Rubio got personal, the U.S. senator from Florida had largely stayed above the fray and had focused his assault on Trump from a policy standpoint.

He told Fox News' "The Kelly File" that if he had to do it over again, he would handle the issue differently.

"My kids were embarrassed by it. My wife didn’t like it. I don’t think it reflects good. That’s not who I am. That’s not what my campaign is going to be about or will ever be about again," Rubio said.

"I’d do it differently – on the personal stuff. I’m not telling you he didn’t deserve it, but that’s not who I am and that’s not what I want to be," he added.

Rubio, 44, fared poorly when Michigan, Mississippi, Idaho and Hawaii voted on Tuesday. He needs to win his home state of Florida next Tuesday or face calls for him to exit the race.

He said opinion polls showing him lagging far behind Trump in Florida were not accurate.

"The fact of the matter is the only poll that counts is the one they’ll take on Tuesday when they count the votes that these people are going to cast and we’re going to win in Florida,” he said.

Cruz, 45, looking to emerge as the main Trump alternative should Rubio and Kasich falter, said he would keep the focus of the campaign on substantive issues.

"I don't have any views on Donald Trump's anatomy," Cruz said at his Fox News town hall.

John Kasich. Fox Business

Brokered convention?

Kasich, at his Fox News town hall, said he believed he had a path forward if he wins his home state of Ohio on Tuesday, but that it involved a convention fight when Republicans gather to officially pick their nominee in Cleveland in July.

A contested convention would result if Trump, 69, does not win the required 1,237 delegates, forcing delegates to decide whether to back him at the convention or find a consensus candidate such as Kasich, 63.

President Gerald Ford and challenger Ronald Reagan staged a spirited nomination fight at the 1976 Republican National Convention, but no Republican convention has gone beyond a single ballot since Thomas Dewey's third-ballot win in 1948.

Trump, who has come under withering attack by mainstream Republicans for his statements on Muslims, illegal immigrants and trade policy, said at his Fox News town hall he had attracted many new voters to the Republican Party with his crossover appeal.

He said establishment Republicans would be risking the energy he had brought to the race by trying to prevent him from winning.

"They would be so foolish to throw it away," he told host Sean Hannity.

(Reporting by Steve Holland; Editing by Peter Cooney)