The budding Republican presidential race has kicked up a notch, after a slew of potential candidates played directly to the base at a forum in Iowa -- an event that saw Sarah Palin back in the mix, Chris Christie proving he can mingle in the heartland, and Ted Cruz and others showing their fiery conservative chops.

Several of the speakers at the daylong Iowa Freedom Summit tilted hard right in their remarks on immigration, security and taxes. And the speakers and audience members showed scorn for those who did not attend -- particularly the more moderate no-shows, like former Florida Gov. Jeb Bush and 2012 GOP presidential nominee Mitt Romney.

As all move toward a decision on a 2016 bid in the coming weeks and months, the event showed battle lines being drawn in what is likely to be a crowded and competitive contest.

Some of the toughest criticism came from real estate mogul Donald Trump, who fired up the crowd by declaring there’s “no way” Bush or Romney could win in 2016.

"You just can't have those two," said Trump, who claims to be, once again, weighing a presidential run.

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Notable was New Jersey Gov. Christie’s ability to win some hearts in the Iowa crowd, not necessarily the core constituency for the East Coast executive.

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The brash governor was quick to dismiss the notion he is “too loud and too blunt” for Iowa.

“If that was the case, why would you keep inviting me back and why would I keep coming back?” he asked, to applause.

“I have been a friend to Iowa, and Iowa has been a friend to me,” he said.

Though he faced an icy reception at first, Christie appeared to win over the crowd, particularly after cracking a joke when an immigration protester started heckling him.

“Don’t they know I am from New Jersey? This stuff doesn’t bother me one bit,” he chuckled.

“I didn’t expect him to be funny,” audience member Lisa Caldroon told FoxNews.com afterward. But would she vote for him in 2016? “Maybe. I liked him a lot more after today. I thought he did a really nice job.”

The Des Moines summit included nearly a dozen Republicans who are at least flirting with a presidential bid, and served as a kick-off of sorts in the run-up to the first-in-the-nation Republican caucuses -- held in Iowa.

The event saw rousing speeches by Wisconsin Gov. Scott Walker, who is attracting some buzz on the heels of the event, as well as Texas Gov. Rick Perry, former Arkansas Gov. Mike Huckabee and others.

For conservative voters, they may give the speakers a closer look. And for their detractors, they saw the attendees catering ever-more to the party’s right flank.

Cesar Vargas, the protester who interrupted Christie, told FoxNews.com he was surprised that Christie and Walker aligned themselves with Rep. Steve King, R-Iowa -- the congressman who organized the event along with Citizens United and is known for his strident remarks against illegal immigrants. (Christie called King a “great friend.”)

"It was very surprising to me," Vargas, who was arrested and then released after his outburst, told FoxNews.com. "We know what to expect from a Steve King or a Sarah Palin. We were very surprised by Walker and Christie, especially.”  

Cruz and Palin, meanwhile, were right at home as they hammered the Obama administration with inventive invective.

Palin in particular has tossed herself back into the primary mix after telling reporters she’s thinking about a 2016 run. She laced her summit address with snappy one-liners as she lit into the current president.

Of Obama, she said: “America, he’s just not that into you.” The 2008 GOP vice presidential nominee also knocked the idea of a Hillary Clinton run.

“Hey Iowa, can anyone stop Hillary? To borrow a phrase, yes, we can!” she said.

And Cruz, the Texas senator, pushed for regulatory and tax reform while comparing the Environmental Protection Agency to locusts.

“The most important tax reform we can do is abolish the IRS,” Cruz told the audience.

He got a huge reaction when he demanded to “repeal every word of ObamaCare.”

But the decision by some several prominent Republicans not to attend -- including Florida Sen. Marco Rubio and Kentucky Sen. Rand Paul, as well as Romney and Bush -- could fuel criticism of their potential candidacies.

Trump said Romney should not be allowed to run a third time and criticized Bush as being weighed down by concerns over his immigration and education policies. Many in the room applauded and cheered following his statements.

Former Pennsylvania Sen. Rick Santorum, who is considering another presidential run, also spoke at the event, as did neurosurgeon-turned-conservative-champion Ben Carson.

Former Hewlett-Packard CEO Carly Fiorina, coming off an unsuccessful Senate run several years ago and weighing her 2016 options, spent her time hammering Obama and other Democrats over everything from the health care law to his immigration policies.