Donald Trump No. 1 target in tonight's Republican debate

Donald Trump may have a big target on his back at the second debate of Republican U.S. presidential candidates on tonight, as his rivals seek to make up ground they have lost to the front-running billionaire with a flair for bomb-throwing rhetoric.

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The debate is being staged and broadcast by CNN from the Ronald Reagan Presidential Library in Simi Valley, Calif.

Many of the 11 candidates who will be at the prime-time debate (5 p.m. PT/8 p.m. ET) see a path for themselves to the Republican nomination for the November 2016 election, and any route they take will require getting out of the long shadow cast by Trump.

For that reason, the gathering, with the stage set up next to the retired Air Force One aircraft used by former president Reagan, figures to be a more combative debate than the first one the Republicans had on Aug. 6 in Cleveland.

Trump had been in the lead in polls of Republican voters at the first debate and grabbed the headlines through a mixture of boastful promises and by ridiculing his rivals, many of whom turned in relatively passive performances.

This time Republicans have a greater incentive to take him on and try to raise doubts about him since Trump has built on his lead, with four months to go until Iowa on Feb. 1 holds the first nominating contest on the road to the November 2016 election.

The latest Reuters-Ipsos opinion poll has Trump leading among Republican voters with 32.2 per cent. Retired neurosurgeon Ben Carson was at 15.8 per cent, former Florida Governor Jeb Bush at nine per cent. The rest of the field below six per cent.

Trump has dismissed the possibility of being attacked at the debate.

Pop quiz

"I hear they are going after me. Whatever. Whatever," Trump said at a lively campaign rally in Dallas on Monday.

CNN correspondents Jake Tapper and Dana Bash will be posing questions, along with conservative radio host Hugh Hewitt.

It was Hewitt who trumped Trump earlier this month. During an interview with the New York businessman, Hewitt gave Trump what amounted to a pop quiz about Islamic militants and other Middle Eastern figures. Trump did not fare well in answering.

The face-off will be the first time Trump has been on stage with former Hewlett-Packard chief executive Carly Fiorina, who made it to the prime-time stage by starring at an undercard event in Cleveland.

Trump last week was quoted by Rolling Stone magazine as saying of Fiorina: "Look at that face. Would anyone vote for that?" He later said he was talking about her "persona."

Fluid race

Fiorina has turned the attack into a vehicle for raising campaign dollars. "Don't you agree that in a presidential campaign, issues matter? Not personal appearance," said a Fiorina fund-raising appeal.

In a sign of how fluid the Republican race is, a CBS News/New York Times poll said 63 per cent of Republican voters had not yet made a final choice on who they would support in the race.

Bush, the Republican establishment candidate once considered a heavy favorite for the nomination, has made clear that if given the chance he will raise questions about Trump's past support of Democratic positions like higher taxes.

At the prime-time debate will be Trump, Carson, Bush, Scott Walker, Ted Cruz, Marco Rubio, Mike Huckabee, Rand Paul, John Kasich, Chris Christie and Fiorina.

Rick Santorum, Bobby Jindal, George Pataki and Lindsey Graham will participate in an undercard debate at 3 p.m. PT/6 p.m. ET).