Cleveland: The Republicans set in motion a key ritual of the American political process on Thursday, with billionaire businessman Donald Trump expected to be the main attraction at the party's first debate of the 2016 presidential campaign.
Trump remains a longshot candidate to replace President Barack Obama, but the brash real estate mogul and reality television star will hold centre stage during the debate in Cleveland, Ohio.
He took that place as top performer in several recent national polls. Only 10 candidates were invited to participate in the main event, with the remaining seven relegated to a pre-debate forum.
It's a key test for Trump, whose unpredictable style and unformed policy positions mean he doesn't fit neatly into any single wing of the Republican Party.
That appears to be a draw to some Republicans frustrated with Washington and career politicians, but others fear his eccentricities and outlandish comments — whether about Mexican immigrants being "criminals" and "rapists" or his questioning of the war record of senator John McCain — will taint the American public's view of the party.
Virtually all Trump's rivals prepared for the debate by having an aide play the man known as "The Donald" during practice sessions, but no one knows how Trump will perform in his first presidential debate.
As in the 2012 Republican primaries, the party faces a tug of war between those eager for a candidate with broad general election appeal and those who think the key to winning is nominating a fiery conservative.
Four years ago, the establishment favoured Mitt Romney, but he struggled to gain the support of conservatives who dominate the state-by-state primary contests that choose the party candidate
Standing to Trump's left on the debate stage Thursday night will be former Florida governor Jeb Bush, a favourite of the wealthy donors and business leaders that populate the establishment wing of the Republican Party. But Bush, the son and brother of two former US presidents, has struggled to separate himself from the rest of the field and he faces questions about whether his nomination would mark a return to the past.
To Trump's right on the stage will be Wisconsin governer Scott Walker, whose victories over labor unions in his home state created his national profile.
Florida senetor Marco Rubio, the youngest candidate in the field at age 44, is trying to carve out a niche as a foreign policy wonk, but has struggled to break through this summer — particularly since Trump's surge.
The remaining seven candidates who didn't make the cut for Thursday night's main debate were relegated to a pre-debate forum, a low-key event in a largely empty arena, where candidates avoided debating each other and largely stuck to scripted responses on domestic and foreign policy.
Former Texas governer Rick Perry and businesswoman Carly Fiorina opened the early event with biting criticisms of Trump.
Perry — whose failed 2012 White House campaign was damaged by an embarrassing debate stumble — accused Trump of using "his celebrity rather than his conservatism" to fuel his run for president.
Fiorina, the only woman in the Republican field, said Trump had tapped into Americans' anger with Washington, but she challenged the businessman as lacking policy positions. "What are the principles by which he would govern?" she asked.
While the candidates pitch their visions for the Republican Party's future, they'll also be making the case that they would present the strongest general election challenge to Hillary Rodham Clinton, the front-runner for the Democratic nomination.
Clinton was scheduled to be traveling during the debate and didn't plan to make a statement afterward. Her campaign was pre-emptively making the case that there was little difference between Trump's "outrageous" positions and the rest of the field.
Thursday's debate is the first of six Republican Party-sanctioned debates scheduled before primary voting begins in February. Fox News, the host of the event, used five national polls to determine which 10 candidates would be on the stage.
The Democratic National Committee, meanwhile, released plans for its presidential debates Thursday, announcing the first of six will be held October 13 in Nevada.
