Former Florida governor Jeb Bush addresses the American Conservative Union’s 42nd annual Conservative Political Action Conference at National Harbor in Maryland. (Pete Marovich/European Pressphoto Agency)

South of the Beltway, at the massive National Harbor hotel and convention center on the east bank of the Potomac, thousands of conservatives have spent the past few days putting Republican presidential candidates-in-waiting through an endurance test.

On Friday, the gantlet was run by a half-dozen hopefuls representing the spectrum of the GOP, from insurgents such as Sen. Rand Paul (R-Ky.) to establishment favorites such as former Florida governor Jeb Bush — all of whom aimed to burnish their credentials with foot soldiers in the conservative movement.

Paul, long a favorite at the annual Conservative Political Action Conference, fired up the crowd with a sharp critique of President Obama’s foreign policy. Others received warm, if less enthusiastic, receptions, including Sen. Marco Rubio (R-Fla.) and former Texas governor Rick Perry.

But for Bush, the moment was critical. He was booed as he came on stage and heckled during a 25-minute appearance. But he pressed his argument that, unlike other invited speakers, he used two terms as Florida governor to implement a conservative agenda that could be adapted nationally.

“Over time, we have to start being for things again,” he said, urging the crowd to support specific policy solutions instead of just campaigning against government programs.

Tea party activist William Temple, from Brunswick, Ga., turns with his flag and leaves in protest as former Florida governor Jeb Bush arrives on stage at CPAC Friday. (Carolyn Kaster/AP)

Cast as a combination festival, recruitment fair and training session, CPAC is sponsored by think tanks, popular conservative Web sites and some of the right’s strongest interest groups, including the National Rifle Association. Current and future GOP politicians, operatives and media figures use the forum to road-test their ideas and rhetorical skills and woo potential volunteers, listeners and donors.

The event is not terribly predictive; the presidential straw poll is often dominated by well-organized Paul supporters. But it is nonetheless viewed as a vital rite of passage for potential candidates in the early phases of the presidential contest and often rewards those who do the most to appeal to the right flank of the party.

Bush opted to use his allotted time to take questions; others gave brief remarks before fielding inquiries from a moderator. Even though dozens of attendees stood and left — some trailing a man dressed in Colonial-era garb — Bush was buoyed by hundreds of supporters, many of whom had been bused to the Prince George’s County convention site from downtown Washington.

Under questioning by moderator Sean Hannity, Bush deflected concerns about his support for national education standards, commonly referred to as Common Core, and for comprehensive immigration reform.

“The simple fact is: There is no plan to deport 11 million people,” he told the room to a mix of cheers and boos. “We should give them a path to legal status, where they work, where they don’t receive government benefits, where they don’t break the law, where they learn English and they make a contribution to our society.”

After his appearance, Bush was asked what he would say to people who walked out of his speech. He turned to a reporter and snapped: “The 10 people who walked out or the 5,000 that stayed?”

Real estate mogul and reality TV personality Donald Trump, who spoke before Bush in the program, tried to paint himself as a Washington outsider ready for the national stage. “I’m not a politician, thank goodness. Politicians are all talk, no action. I’ve dealt with them all my life.”

But in a question-and-answer session with Hannity, Trump fell into a familiar pattern: questioning whether Obama was born in the United States:

“Hey, look, [the president] wrote a book when he was a young man, and it said ‘born in Kenya,’ blah, blah, blah. I don’t know where he was born. I would like to see his college records. I think that’s important.”

During his speech, Trump covered a long list of topics, including his opposition to Common Core standards, his support for a tougher position against Iran and for infrastructure development.

The business mogul was most aggressive in his criticism of the administration’s management of the fight against the Islamic State, saying the president “doesn’t know what he’s doing.”

“On ISIS, nobody, if I decide to run and win, nobody would be tougher than Donald Trump. I would hit them so hard and so fast that they wouldn’t know what happened.”

Paul, a popular libertarian-leaning senator whose father, former congressman Ron Paul, was once a favorite of the CPAC crowd, sought to cast national defense as a top priority. He said he doesn’t think that the United States should be “unnecessarily meddling” overseas but added that the country must defend itself from the Islamic State terror group.

“We must protect ourselves from jihadists without losing ourselves as a people,” he said, adding he envisioned “a national defense unparalleled, undefeated and unencumbered by nation-building.”

Paul also hammered former secretary of state Hillary Rodham Clinton — the presumptive 2016 front-runner on the Democratic side — saying that her handling of the 2012 attack on a U.S. diplomatic compound in Libya amounted to “dereliction of duty.”

“It’s time for Hillary Clinton to permanently retire,” he said.

Rubio, a rising star who nonetheless faces suspicion among many conservatives over his role in fashioning immigration legislation, emphasized his distance from the border plan he once backed. He blamed the Obama administration for the failure of a 2013 Senate immigration bill to move ahead in the House, which never considered the measure.

The first-term senator also received a standing ovation when he jabbed at Obama’s foreign policy and delivered a robust defense of “American exceptionalism.”

“Sometimes you wouldn’t know we’re an exceptional nation by listening to the president, who’s described our nation as sometimes being arrogant or dictating terms to others,” he argued. “But Americans know we’re exceptional, and you know who else knows we’re exceptional? The world does. After all, when was the last time you heard about a boatload of Americans arriving on the shores of another country?”

Perry used his speech to recast himself after a short and disastrous 2012 presidential campaign. He peppered his speech with foreign and domestic policy nuances, intended to showcase a broad knowledge of issues.

“The weakness and incompetence of our government shouldn’t be confused with the strength, the ingenuity and the idealism of the American people,” he said.

The conference concludes Saturday, when organizers will announce the results of the straw poll of attendees. Paul has won the contest for the past two years.

Several attending Friday’s session said they were first-time attendees. Most left on Friday undecided on who to support for president.

Craig Dalton, 21, a college student at Louisiana State University and a supporter of Paul, joined the group who left Bush’s speech in protest.

“Common Core, I don’t like that in any way,” Dalton said, tapping on his lapel’s “Stand with Rand” sticker. “Bush is against legalization of marijuana, too, even though he has admitted to smoking it when he was young!”

Rick McLaughlin, 55, a financial adviser from Broad Run, Va., attended CPAC for the first time with his daughter, Abigail. He said he was favoring Paul over Bush.

“I thought there was some double-talking, and he didn’t answer the questions completely,” McLaughlin said of Bush. “Compared to Rand Paul, who was very direct and clear-cut. It felt like it was a comparison between a politician versus a genuine guy.”

Robert Costa, Katie Zezima, and Abby Livingston of the Texas Tribune contributed to this report.