A woman walks past a caricature of Republican presidential candidate Donald Trump before the start of the second official Republican presidential candidates debate of the 2016 U.S. presidential campaign at the Ronald Reagan Presidential Library in Simi Valley, California, September 16, 2015. [Photo/Agencies]

SIMI VALLEY, Calif.- Low-polling Republican presidential candidates attempted to cast doubt on lead rival Donald Trump on Wednesday, venting their frustrations at the state of the 2016 race by calling the billionaire a fake conservative with a checkered business past.

Louisiana Governor Bobby Jindal and former New York Governor George Pataki led the charge against Trump at a debate of four candidates who did not have enough strength in opinion polls to make it to a prime-time debate later.

The so-called "undercard" debate at the Ronald Reagan Presidential Library of four candidates immediately preceded the main event featuring Trump and 10 other Republicans who are doing the best in the polls.

In their bid to stop Trump's rise, a variety of Republican candidates have piled on the billionaire businessman as a politician who has supported Democratic priorities in the past, such as higher taxes. Jindal, one of Trump's sharpest critics, quickly picked up on that theme.

"Let's stop treating Donald Trump like a Republican," Jindal said in response to a question about his criticism of the Republican front-runner.

"He's not a conservative. He's not a liberal. He's not a Democrat. He's not a Republican. He's not an independent," Jindal said. "He believes in Donald Trump."

Pataki was similarly biting in his criticism of Trump, whose unexpected rise in the polls has put pressure on the other 15 Republican candidates to respond. Pataki has vowed not to support Trump if he is the nominee for the November 2016 election but insisted he does not think Trump will get that far.

"Donald Trump is not going to be the Republican nominee, period. I guarantee you that," said Pataki.

He said Trump's experience in the casino business in Atlantic City, New Jersey, was instructive of the type of businessman he is.

"Every one of those casinos went bankrupt, over 5,000 Americans lost their job," Pataki said. "He didn't lose anything. ... He will do for America what he did for Atlantic City and that is not someone who we will nominate."

Tempers flared as the candidates debated what to do about illegal immigration, Trump's signature issue.

Senator Lindsey Graham of South Carolina, who has backed comprehensive immigration reform, said it would be impractical to try to deport 11 million illegal immigrants.

He mocked former Senator Rick Santorum's own failed attempt at an immigration plan when Santorum was still in office in 2006.

"It went nowhere," Graham told Santorum.

The former senator from Pennsylvania shot back: "You're right, Lindsay, it went nowhere because we had a president back then who was for more comprehensive immigration reform than I was," Santorum said.

Responded Graham, "George W. Bush! Who won with Hispanics!"

Santorum replied, "We need to win fighting for Americans."

"Hispanics are Americans," Graham said, getting applause.

All four candidates declared Democratic President Barack Obama's handling of Islamic State militants in Syria and Iraq as a disaster.

Graham was insistent in saying the United States should raise its troop level in Iraq from 3,500 to 10,000 to more effectively take on Islamic State, and called for sending American troops into Syria as part of a regional army. He expressed concern about the ability of militants to strike targets within the United States.

"We're in a war, folks," he said.