Watch CNN’s coverage of the fifth Republican presidential debate live from Las Vegas on Tuesday, December 15. Coverage begins at 6 p.m. ET.

Story highlights

Donald Trump is now at 41% nationally in a new poll, but trailing or tied in Iowa with Ted Cruz

Trump's proposal for a temporary ban on Muslims from entering the United States has seemingly woken up the GOP establishment

Las Vegas, Nevada CNN  — 

Donald Trump – the billionaire businessman who is dominating the Republican presidential campaign – is suddenly on defense like never before.

As he prepares to take center stage in Tuesday’s CNN Republican presidential debate, Trump is under fire from party leaders – and many of his White House rivals – for advocating a temporary ban on Muslims from entering the United States. And after cruising at the top of the polls for five months, Trump now faces sharp competition in Iowa, where Ted Cruz has taken the lead in some polls.

Trump, who has shown an uncanny ability to withstand controversies that would have felled other politicians, will no doubt set the tone during the two-hour showdown Tuesday night.

“He’s been surfing the wave for months. He’s never fallen off the wave,” said Doug Heye, a former spokesman for the Republican National Committee. “He’s figured out how to keep the wave going: Every eight or nine days, say something ‘outrageous.’”

Read: Trump’s doctor: Trump ‘will be healthiest individual ever elected’ president

Tuesday marks the last GOP debate of 2015, a critical opportunity for each White House hopeful to make a lasting impression with voters before the year-end holiday lull. It is also one of the final public opportunities to do something – anything – to try to shake Trump off of his top perch before voting begins.

Speaking at a campaign rally Monday night at the Westgate Resort & Casino, Trump touted his continued dominance in national polls and predicted that he would be a favorite target on the debate stage.

“They’re all coming after me,” he said. “This will not be an evening in paradise for me.”

Even on defense, however, he remains the undisputed national GOP front-runner, with a Monmouth University survey Monday placing him at an astonishing 41% among Republican voters. That’s the first time he’s cracked the 40% threshold in a national survey, though a Washington Post/ABC News poll out Tuesday showed Trump at 38%.

Pressure on rivals to knock Trump off his game

As he’s come under unprecedented criticism, Trump is also once again dangling the threat of an independent presidential run, reprising what was an issue at the first GOP debate back in August. The first question then to the candidates: is anyone unwilling to pledge not to run as an independent? Only Trump raised his hand.

Now, other candidates – and key Republican lawmakers – are being asked the same question: Would they back Trump should he win the nomination?

Asked that question in New Hampshire last week, Jeb Bush said it won’t be an issue. “That’s a hypothetical I reject out of hand,” Bush said.

Bush added that pundits are “obsessed about” the idea of Trump winning, but believes voters will go in another direction.

Yet Trump isn’t going away and continues to push the GOP conversation. Following the terrorist attacks in Paris in November and a mass shooting in San Bernardino, California, this month, Trump has made a forceful case for why he is best-equipped to fight ISIS and curb terrorism. But while some of his rivals have proposed strengthening the screening process for refugees entering the country, Trump put forth an idea that his critics have labeled fascist: a “total and complete shutdown of Muslims entering the United States.”

That proposal jolted the Republican establishment.

Read: How Donald Trump took the Republican Party by storm

And Trump’s rhetoric and continued national success means the pressure is also on for all of his rivals.

“I’d be happy if he comes out holding his own. He’s the front-runner. He just needs to sustain it,” said longtime Trump associate Roger Stone, who is no longer officially involved with the campaign but says he still periodically speaks with the candidate. “The pressure’s on the other guys. The other guys are the ones who are lagging.”

Here comes Cruz

One rival is closing in on Trump.

Ahead of the CNN debate, Cruz – who is making an appeal to his party’s most conservative base and voters disenchanted with the GOP establishment – has started to overtake Trump in some Iowa polls.

A Des Moines Register/Bloomberg Politics survey gave Cruz a significant lead over Trump, 31% to 21%, while a Fox News poll showed the Texas senator ahead in a tighter match-up against Trump, 28% to 26%. A Quinnipiac University poll Monday, however, showed Trump slightly ahead of Cruz, 28% to 27%.

And although Trump is still in first place nationally, there have been signs of Cruz gaining ground nationwide as well. An NBC News/Wall Street Journal poll out Sunday had Trump at 27% and Cruz in second place with 22% support. The Monmouth poll, however, has Cruz back at 14% and Marco Rubio at 10%.

Trump has finally decided to go negative on Cruz, launching multiple attacks at the senator including several aimed at specifically appealing to Iowa conservatives.

“You look at the way he’s dealt with the Senate, where he goes in there like – frankly like a little bit of a maniac,” Trump said of Cruz over the weekend on Fox News.

In a fundraising pitch to supporters Sunday, Cruz lamented that he’s “about to be attacked like never before.” “I am already facing the most brutal attacks since I announced my campaign,” he wrote.

But despite coming under fire, Cruz is unlikely to respond in kind to Trump at Tuesday’s prime-time event.

The senator has exercised strict restraint so far when it comes to the party’s favorite punching bag, publicly remarking that he has no interest in getting into a cage match with his famously vengeful rival and making a point of regularly praising Trump.

Kellyanne Conway, a Republican pollster working with a pro-Cruz super PAC, said Trump’s rivals would be better served by simply ignoring the bombastic frontrunner and focusing on their own key issues, instead.

“Nobody’s going to have a silver bullet, a magic elixir on stage at the CNN debate that dislodges Trump from the top of the polls. That’s a fool’s errand,” Conway said. “It’s incumbent upon these candidates to show contrast from other people’s point of views, including Mr. Trump’s.”