There will be a few empty seats around one Thanksgiving table this year: Donald Trump and former Florida Gov. Jeb Bush are among those skipping Friday's Presidential Family Forum in Des Moines. The event is hosted by the conservative Christian group the Family Leader. President Bob Vander Plaats is seen as an evangelical kingmaker in Iowa.

Discussions of politics and religion may be ill-advised at most holiday feasts, but not so here. Instead, Republican presidential hopefuls sit together around a table and answer open-ended questions that Vander Plaats says are designed to reveal their "character."

Vander Plaats has a track record of endorsing Iowa Caucus winners — including former Pennsylvania Sen. Rick Santorum in 2012 and former Arkansas Gov. Mike Huckabee in 2008. He says it's a mistake for candidates to skip the forum. Vander Plaats says he spoke to Trump this week, and that the canddiate offered some conditions.

"He would like for me to guarantee my endorsement to go to him, and obviously he would show up," Vander Plaats said. "And I like Mr. Trump a lot, because the one thing I really value about Mr. Trump is he's very candid, he's very authentic, and he's very real. So I appreciate that about him of saying, 'I'm gonna go to another event if I can't guarantee your endorsement.'"

Still, Vander Plaats called Trump a "tremendous candidate," who has pushed the Republican Party to talk about "some very big issues." He said Trump will need the evangelical base to support him a year from now if he becomes the Republican nominee.

Asked for comment, Trump's campaign spokesperson said he has had a longstanding engagement in South Carolina. He's scheduled to campaign in Spartanburg, S.C., midday Friday, and in Birmingham, Ala. the next morning.

The format for the dinner — a soul-searching discussion with a heavy focus on faith –- isn't Trump's strong suit.

Sometimes, the questions lead to emotional discussions of personal hardships.

"That's when we know what does a candidate really believe? What's inside their heart?" Vander Plaats said

On Friday evening, those kinds of questions will be posed to seven candidates by the moderator, conservative political consultant Frank Luntz. (Eight were confirmed before Louisiana Gov. Bobby Jindal dropped out).

It's worth noting that Trump made one of the most controversial statements of his campaign in July, at another Family Leader event. Speaking at the Family Leadership Summit, Trump criticized Arizona Sen. John McCain, telling the audience, "He's a war hero because he was captured. I like people that weren't captured."

But it's Jeb Bush, Vander Plaats said, who's "making the biggest mistake" by not attending. He points out that evangelicals were important in helping Bush's father and brother win their own elections, and says his struggling campaign should be working harder to win them over now.

"There's a lot of Bush ties here in Iowa, and so not to come to a forum like this is sending, I think, the wrong message," Vander Plaats said.

Bush campaign spokesman Tim Miller says Bush is unable to make the event because he'll be leading a , an event Miller said he's been involved in for 20 years.

Vander Plaats said he plans to announce an endorsement in the GOP primary sometime between Thanksgiving and Christmas — and the Family Leader's board may also weigh in with its own pick. He says there's a good chance he'll choose from among the candidates who gather around the table in Des Moines on Friday evening.

"There's always that possibility that we could endorse somebody who doesn't show up," Vander Plaats said. "But I'm a former educator. This is the final exam. And typically students that skip the final exam don't do very well."

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