The Republican presidential candidates participating in Thursday’s debate, from top left, Jeb Bush, Ben Carson, Chris Christie, Sen. Ted Cruz, Mike Huckabee and from bottom left, John Kasich, Sen. Rand Paul, Sen. Marco Rubio, Donald Trump and Scott Walker. (AP)

Jeb Bush has been practicing all summer for this moment: his first big chance to convince voters he is a reliable problem-solver fit to sit in the Oval Office.

Scott Walker has been boning up on issues foreign and domestic, eager to prove that after a rocky debut this spring he has amassed the knowledge to be a credible commander in chief.

Ted Cruz spent this week ensconced with advisers inside a Capitol Hill townhouse honing the rhetorical and theatrical skills that made him a standout on the college debate circuit.

But as the largest GOP field ever takes the stage here Thursday night for the hotly anticipated first presidential debate, all eyes will be on the unexpected front-runner, Donald Trump, who, like any good showman, is promising to perform in unscripted glory.

The potentially combustible Fox News Channel debate, which begins at 9 p.m. Eastern time in the arena home of LeBron James and the Cleveland Cavaliers, features Trump as the wild card. Will he hold his own amid tough questioning from the moderators? Will he draw blood from his opponents? Will he commit a mistake that deflates his candidacy, or will he appear presidential and broaden his base of support?

Presidential debate season is underway, and our 2016 candidates should take a few lessons from the past. The Fix's Chris Cillizza revisits the 2012 Republican debates and some of its most "oops" moments. (Julie Percha/The Washington Post)

By virtue of his polling lead, Trump will be positioned at center stage, flanked by Bush and Walker, who have averaged second and third, respectively. The rest of the top 10 candidates will fan out from there, while the remaining seven candidates will participate in a 5 p.m. undercard debate. (One of the seven, Carly Fiorina, dubbed it the “happy-hour debate.”)

The Cleveland showdown opens a new phase of direct combat for the Republicans. A debate once envisioned as an introductory forum has become an un­predictable drama, thanks to Trump’s rise. Millions of voters are expected to tune in — and most of the candidates have privately acknowledged the high stakes.

“Debates are great levelers,” said veteran GOP consultant Alex Castellanos. “There is a physics to this. . . . Somebody is going to have a big moment where they show their best and true self.” Everyone wonders, he added, “Who is that person going to be?”

Trump intends to make the case that it’s him. In an interview, Trump previewed a wait-and-see approach. He has spent this week at his gleaming Manhattan skyscraper, Trump Tower, basking in news articles about his sustained lead in the polls rather than memorizing zingers and staging mock debates.

“I’ll have to feel it out, see where everyone else is coming from,” Trump said. “I’d prefer no conflict, no infighting, but if they hit me, I’ll hit them harder. It’s all going to depend on the moment.

“You watch,” he continued. “I am going to keep it on a high level. I have a lot of respect for them.”

The Cleveland debate is the first of nine nationally televised primary debates sanctioned by the Republican National Committee, to be held roughly once a month through early next year.

In the 2012 campaign, primary debates fueled the rapid rise and collapse of several candidates and ultimately damaged the eventual Republican nominee, Mitt Romney. This time around, with 17 declared candidates and no dominant front-runner until Trump’s summer surge, the debates provide arguably the best opportunity for contenders to break through.

Although just a single August evening in a protracted race, Thursday’s debate here on the shores of Lake Erie — at the Quicken Loans Arena, which next summer will host the Republican National Convention — will begin to set the tone ahead of next February’s first caucuses and primary elections.

For now, Trump’s catapulting candidacy is the dominant story line. One of the debate moderators, Chris Wallace, said Tuesday evening on Fox, “We don’t want to make it the Donald Trump show, but it is.”

Democratic front-runner Hillary Rodham Clinton and her campaign have tried to use Trump’s inflammatory statements, especially on immigration, to paint the entire GOP field as extremists.

“I understand Trump may be an irresistible focal point,” Joel Benenson, Clinton’s chief strategist and pollster, told reporters Wednesday. He said the real question to be asking during the debate is, “Do the Republicans do anything to suggest they are anything other than out of touch and out of date?”

Clinton is expected to be a focal point for the Republicans, though Benenson shrugged off the suggestion that the two-hour debate poses a risk to her.

Some leading Republicans will be trying to avoid confrontation with one another. Bush hopes to convey his policy substance and explain his conservative record as Florida’s governor.

“It comes down to this: Are you going to be angry or a problem-solver?” said Al Cardenas, a Bush friend and ally. “Voters want to see someone with presidential timber. . . . I doubt Jeb attacks anyone. If anything, he’ll respond to something if it’s over the top.”

Cardenas said Bush didn’t need to do much cramming, although he recently called two of Romney’s longtime advisers and debate gurus, Beth Myers and Peter Flaherty, to the Bush family compound in Kennebunkport, Maine, for prep sessions.

For Walker, the governor of Wisconsin, the goal is to come across as the nice guy with an optimistic vision for the future.

“I’m not going to worry about the other candidates,” Walker said during a campaign stop Monday at a pizza parlor in New Hampshire. “People are tired of politicians telling you who they’re against and what they’re against. . . . Americans want to vote for something and for someone.”

Another contender, Sen. Marco Rubio of Florida, on Wednesday afternoon drew hundreds of mostly younger Republicans to TownHall, a “vegan-friendly eatery” in Cleveland’s Ohio City neighborhood. He called the race a “generational choice” and bantered in Spanish with the crowd. “Tune in tomorrow night,” he said.

Cruz, who was a star debater at Princeton University, huddled Tuesday and Wednesday with his inner circle at Hillsdale College’s townhouse in Washington to get ready for the debate. Helping the senator from Texas prepare was Anthony R. Dolan, one of President Ronald Reagan’s speechwriters.

In an interview Tuesday, Cruz said his strategy sessions have concentrated on “how one communicates with 10 somewhat conflicting messages all on the same stage at the same time.”

Ohio Gov. John Kasich last week held a mock debate session at his home in Columbus and has been reviewing a five-page memo of possible questions his staff prepared. He is relieved to be in the main debate at all, inching past former Texas governor Rick Perry in the latest polls to score the 10th spot.

The two candidates who have been the most aggressive in taking on Trump — Perry and Sen. Lindsey O. Graham of South Carolina — won’t face him on stage. Their low polling relegated them to the junior-varsity debate.

One candidate on the prime-time platform who might take some shots is New Jersey Gov. Chris Christie. He confidently previewed his debate performance on Tuesday night in Asbury Park, where his donors feted him with a “kickoff reception.”

“We all want to see Chris get up there and be himself,” said Ray Washburne, Christie’s national finance chairman. “Our message to him all along has been, ‘Trump will be Trump, you be you — and sometimes, well, a little rough-and-tumble can work.’ ”

Anne Gearan and Katie Zezima in Washington and Jenna Johnson in Manchester, N.H., contributed to this report.