If Mr. DeSantis ends up doing so, Mr. Christie said he had one piece of advice: “Get the hell out of the race.”

In an interview this month during his surprise visit to Ukraine, Mr. Christie said he was not particularly bothered by the prospect of a debate without Mr. Trump.

“It doesn’t change my perspective or my tactical approach,” he said. “Because if he’s not there, it just means two things. One, he’s afraid to be on the same debate stage and defend his record. And two, you know, this is a guy who, by not showing up, just gives me more time. So it’s OK. Either way, I win.”

Mr. Christie and his advisers see the debate as a forum built to the former governor’s strengths. Comfortable in unscripted moments in front of cameras, Mr. Christie has a confrontational style, hewed from years in the trenches of New Jersey politics, that has served him well in past debates, and he has over a decade of experience participating in them through two campaigns for governor and his 2016 run for president.

To prepare for Wednesday, he has been huddling with close advisers to go over topics at the heart of the campaign and anticipate different scenarios that may arise as he navigates the chaos of an eight-lectern stage. He has been heavily focused on the debate, bringing it up in casual conversations with both advisers and political acquaintances as he takes the temperature of the race.