LAS VEGAS (Reuters) - Ted Cruz has avoided direct confrontations with Donald Trump throughout the New York billionaire’s long run as the Republican front-runner for the 2016 presidential nomination. That strategy will be put to the test at Tuesday night’s Republican debate.

Trump, who has seen Cruz surge ahead of him in polls in the early voting state of Iowa, has begun an onslaught against the U.S. senator from Texas and could ratchet up his attacks at the nationally televised forum in Las Vegas.

Trump called Cruz a “maniac” on Sunday, an indication that he is prepared to take aim at Cruz as he has successfully done against rivals Ben Carson, Jeb Bush, Rand Paul and John Kasich.

Cruz will face a choice: Return fire and be drawn into a mud fight or sidestep the attacks and risk looking weak.

As Trump and Cruz circle each other, the moderators for CNN, which is hosting the debate, are likely to press the nine top-polling Republican candidates sharing the stage about Trump’s comments last week calling for a ban on Muslims entering the United States. Trump’s remarks stirred international outrage, and other candidates blasted him.

But Trump has neither backed down nor has he dropped in national polls. The latest Reuters/Ipsos poll, completed after Trump’s comments, showed him leading the field with support of 35 percent of Republican voters. Carson was second with 12 percent, followed by Cruz and former Florida Governor Jeb Bush, tied at 10 percent.

Cruz, whose slicing rhetoric and tactics have made him few friends in the U.S. Senate, is emerging as a leading rival to Trump for the Republican nomination for the November 2016 presidential election thanks in part to his strong organization in Iowa and large campaign war chest.

Republican U.S. presidential candidate Donald Trump gives a thumbs-up during a speech at a Donald Trump for president rally in Las Vegas, Nevada December 14, 2015. REUTERS/Mike Blake

If Cruz wins Iowa’s caucuses on Feb. 1, he could be well-placed for a strong finish in New Hampshire’s first-in-the-nation primary on Feb. 9, and potentially a victory in South Carolina, next up on the primary calendar, where his conservative message plays well.

All this should give Trump the incentive to go on the offense at the debate, which will begin at 9 p.m. EST on Tuesday (0200 GMT Wednesday).

“If you’re a threat to him, he’ll go after you,” said Republican strategist Rick Wilson. “And as much as Cruz remains an acceptable figure for a lot of voters, the minute Trump goes after him he’ll be thrown off the island in a hot second.”

More than any of his rivals, Cruz has steered clear of public fights with Trump, or as Wilson described it, Cruz has been “like a pilotfish to Trump’s very large white shark.”

Cruz’s supporters say the strategy has worked. They are cautioning Cruz, who honed his debating skills as a student at Princeton University, to resist the temptation to take on Trump now.

“Everybody is saying he should go after him,” said Saul Anuzis, a former chairman of the Michigan Republican Party who is a Cruz backer. “My preference would be for him to stay on his own course and not get sucked into other people’s efforts to take one or the other candidates down.”

But a failure to swing back at Trump on national TV could make Cruz look weak at a time when voters appear to be throwing their support behind aggressive, outspoken candidates.

Trump’s supporters seem uninterested in criticism of their candidate’s tough views. A rally in Las Vegas on Monday grew tense as protesters interrupted several times and had to be forcibly removed. Video obtained by NBC News and others captured people in the crowd yelling expletives and a Nazi-era slogan at the protesters.

Trump, who is often interrupted at campaign events by people protesting his views on immigration and other topics, encouraged his supporters to stage their own protests. “We should have been doing what they’re doing for the last seven years,” he told the crowd, in a criticism of the Obama administration.

The CNN debate is the last such Republican encounter of 2015, and as such, candidates want to leave an impression before voters settle in for the Christmas holiday. That means Trump could draw heavy fire.

“If you want to win, place or show in Iowa and New Hampshire, you have to attack Trump in the Vegas debate and show some chops,” said Republican strategist Scott Reed.