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Front-runners Donald Trump and Ted Cruz are expected to clash, breaking their months-long entente in the last Republican presidential debate of 2015.
It is the first Republican debate since Mr Trump urged the US impose a ban on Muslim arrivals, in light of the California terror attacks.
The plan was condemned by his White House rivals and several world leaders.
New York billionaire Mr Trump is the frontrunner but Texas Senator Ted Cruz has surged in recent weeks.
Tensions have recently increased between the two men who have made pains to avoid criticising each other.
National security is expected to be a central theme of the night, two weeks after a radicalised Muslim couple killed 14 people in California, and just over a month since the attacks in Paris.
One of the suspects in the California attack, Tashfeen Malik, professed support for Islamic State militants on social media.
The attack in Paris brought the issue of Syrian refugees to the front of the US political debate. That debate expanded to include the entry of all Muslims to the US with Mr Trump's proposed ban following the attacks in California.
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Mr Trump's proposal generated global criticism, and in its wake, Mr Cruz has tried to cast himself as a more electable alternative.
The Texan senator has a reputation for clashing with his own party and for his prickly personality.
The two have avoided open and direct clashes for months, but the friendly relationship has shown cracks in recent days.
Meanwhile, Florida Senator Marco Rubio is working to pitch himself as the favoured alternative to both men - capable of capturing both the Republican establishment voters and those who crave a so-called "outsider".
Mr Rubio, a Cuban-American like Mr Cruz, has tried to paint his fellow senator as a political opportunist who is weak on national security and foreign policy.
Image copyright Getty Images Image caption Mr Rubio, trailing both, is hoping to present himself as an electable alternative to Messrs Trump and Cruz Mr Cruz's campaign has roundly denied the allegations, with a spokesman saying "nobody believes Senator Cruz is weak on national defence and security".
The debate will also prove crucial for two candidates who have so far struggling in the polls: Florida Governor Jeb Bush and New Jersey Governor Chris Christie.
Mr Bush, whose campaign is flush with cash but strained when it comes to support, is looking to head off calls that he should drop out of the race; those calls have been growing louder in recent weeks.
Mr Christie did not enjoy early support, but appears to be gaining ground as the other governor loses it. In the important primary state of New Hampshire, he's captured important endorsements and appears to be gaining in polls in recent weeks.
Image copyright Getty Images Image caption The luxiour Venetian Hotel will play host to the Republican debate Also on the stage, will be Ben Carson, who is struggling to regain his footing after a series of missteps on foreign policy issues.
The main debate featuring the top eight Republican candidates will start at 2100e (0200 GMT), after a so-called undercard debate featuring those in the lower ranks.
The state-by-state primary contests in the presidential election begin in six weeks in Iowa on 1 February and will last for months.
Each party will formally nominate their candidate over the summer, with Hillary Clinton the favourite to win the Democratic nomination.
Americans will finally go to the polls in November, and the newly elected president will assume office in late January of 2017.
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It is the first Republican debate since Mr Trump urged the US impose a ban on Muslim arrivals, in light of the California terror attacks.
The plan was condemned by his White House rivals and several world leaders.
New York billionaire Mr Trump is the frontrunner but Texas Senator Ted Cruz has surged in recent weeks.
Tensions have recently increased between the two men who have made pains to avoid criticising each other.
National security is expected to be a central theme of the night, two weeks after a radicalised Muslim couple killed 14 people in California, and just over a month since the attacks in Paris.
One of the suspects in the California attack, Tashfeen Malik, professed support for Islamic State militants on social media.
The attack in Paris brought the issue of Syrian refugees to the front of the US political debate. That debate expanded to include the entry of all Muslims to the US with Mr Trump's proposed ban following the attacks in California.
The Republican contest - in debate
Image copyright AFP Image caption The Venetian Hotel in Las Vegas has been transformed for the debate How to dump Trump - four ways his rivals might beat him at the debate
Muslims on Trump - Muslims in Las Vegas reflect on Trump ban plan
Cruz on the rise - five things to know about the Texas senator
Trump's chances - could he really win the White House?
Mr Trump's proposal generated global criticism, and in its wake, Mr Cruz has tried to cast himself as a more electable alternative.
The Texan senator has a reputation for clashing with his own party and for his prickly personality.
The two have avoided open and direct clashes for months, but the friendly relationship has shown cracks in recent days.
Meanwhile, Florida Senator Marco Rubio is working to pitch himself as the favoured alternative to both men - capable of capturing both the Republican establishment voters and those who crave a so-called "outsider".
Mr Rubio, a Cuban-American like Mr Cruz, has tried to paint his fellow senator as a political opportunist who is weak on national security and foreign policy.
Image copyright Getty Images Image caption Mr Rubio, trailing both, is hoping to present himself as an electable alternative to Messrs Trump and Cruz Mr Cruz's campaign has roundly denied the allegations, with a spokesman saying "nobody believes Senator Cruz is weak on national defence and security".
The debate will also prove crucial for two candidates who have so far struggling in the polls: Florida Governor Jeb Bush and New Jersey Governor Chris Christie.
Mr Bush, whose campaign is flush with cash but strained when it comes to support, is looking to head off calls that he should drop out of the race; those calls have been growing louder in recent weeks.
Mr Christie did not enjoy early support, but appears to be gaining ground as the other governor loses it. In the important primary state of New Hampshire, he's captured important endorsements and appears to be gaining in polls in recent weeks.
Image copyright Getty Images Image caption The luxiour Venetian Hotel will play host to the Republican debate Also on the stage, will be Ben Carson, who is struggling to regain his footing after a series of missteps on foreign policy issues.
The main debate featuring the top eight Republican candidates will start at 2100e (0200 GMT), after a so-called undercard debate featuring those in the lower ranks.
The state-by-state primary contests in the presidential election begin in six weeks in Iowa on 1 February and will last for months.
Each party will formally nominate their candidate over the summer, with Hillary Clinton the favourite to win the Democratic nomination.
Americans will finally go to the polls in November, and the newly elected president will assume office in late January of 2017.
Share this story About sharing
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