CLEVELAND -- Donald Trump got laughs and some boos, Rand Paul was the most aggressive and Chris Christie tried to out-Trump Trump in the first debate among Republican presidential candidates here Thursday night.

The 10-man forum began with fireworks from the first question -- when Trump refused to pledge to not run as an independent -- and rollicked with small skirmishes between candidates looking to either distance themselves from rivals or knock poll leaders down a peg.

It was the first step in what will be a long slog for one of them -- or one of the seven other candidates who were relegated to an earlier drive-time debate -- to earn the GOP nomination next year at the convention in this city. It was also the first major test for contestants to try to break out of a crowded field.

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Christie, the New Jersey governor, and Paul, a Kentucky senator, battled over the Patriot Act and collection of private citizens' data. Former Florida Gov. Jeb Bush and Sen. Marco Rubio (R-Fla.) sought to thump Trump on immigration.

Ohio Gov. John Kasich was the lone contestant to voice support for treating gay Americans "with respect" and say expanding drug treatment was fiscally conservative because it reduced long-term costs. He said Trump's immigration comments "hit a nerve" with Americans because "people are frustrated. They're fed up."

Former Arkansas Gov. Mike Huckabee said he'd invoke the 5th and 14th amendments to essentially stop all abortions. Bush said his brother, ex-President George W. Bush, in hindsight made a mistake by invading Iraq.

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"Knowing what we know now . . . it was a mistake," Jeb Bush said, although he added: "ISIS was created when we left."

Much of the pre-debate anticipation focused on Trump, the front-runner in most polls. And Fox News moderators put him on the spot with the opening question, asking anyone to raise his hand if he can't, at this time, pledge to support the eventual GOP nominee.

Trump put up his hand and, when pressed, said he wouldn't support the nominee if it was someone he couldn't respect -- though he didn't name any.

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Some in the audience booed, and Paul jumped right in and said Trump wouldn't pledge because he was "used to buying" politicians. At another point, Bush noted he had criticized Trump before for using "divisive" language.

And it was on.

Trump got the biggest laughs. When moderator Megyn Kelly asked him about his reported disparaging remarks about women he has made, using epithets such as "fat pigs" or "slobs," Trump interrupted and said: "Only Rosie O'Donnell."

He added that he wasn't "politically correct" and that being politically correct was a "big problem in this country" and bristled at Kelly: "I've been very nice to you, although I could probably maybe not be, based on the way you have treated me."

Paul got into it with Christie over the Patriot Act. Paul has criticized data collection by the Obama administration and questioned whether the law was too broad, infringing on law-abiding citizens.

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Christie, a former federal prosecutor, said investigators couldn't know who was a potential terrorist without broad collection powers.

"I stand for the Bill of Rights," Paul declared.

"That's a completely ridiculous answer," Christie countered.

Paul blasted him for hugging Obama after superstorm Sandy slammed the Northeast, saying: "I don't trust Obama with our records. I know you gave him a big hug, and if you want to give him a big hug again, go right ahead."

The candidates spoke at length on immigration. While all said they wanted greater border control, Bush, whose wife is from Mexico, said most people who illegally cross the border are trying to provide for families back home.

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Bush tried to push back at critics who say he is among the leaders in the polls because of his family name, contending that it works against him.

"Maybe the bar is higher for me. That's fine," he said.

Florida Sen. Marco Rubio had to defend his record for supporting some anti-abortion bills that provided exceptions for incest and rape and some that didn't.

Sen. Ted Cruz (R-Texas) used more of his time on foreign affairs than any other candidate, according to news outlets that tracked it, repeatedly attacking Obama.