With only 24 hours remaining before a Republican presidential debate in Des Moines, Iowa, Fox News Channel host Bill O'Reilly tried everything he could think of during a Wednesday interview to convince front-runner Donald Trump to reconsider his boycott. O'Reilly appealed to Trump's pride, political strategy and Christian virtue. In fact, our Jenna Johnson broke down the whole case into 14 parts.

Toward the end of the conversation, when all else had failed, O'Reilly summoned his inner Kelis and made one final argument: "I bought you so many vanilla milkshakes. I bought you so many vanilla milkshakes — you owe me. Will you just consider? I want you to consider, all right? Think about it."

O'Reilly offered to forgive Trump's milkshake debt if he agreed to attend Thursday's debate, which Fox News will air. The proposal got a chuckle out of Trump, who acknowledged that he does indeed owe O'Reilly some milkshakes, but the billionaire appeared unmoved. He said he would rather pay off the debt than show up.

So that was ... different?

Yet this odd exchange might tell us as much about Trump as anything else we've seen in his campaign. O'Reilly's milkshake argument — and its ineffectiveness — is a prime example of just how strong Trump's independent streak really is. We know that the real estate mogul has little regard for the advice of political pundits, but he and O'Reilly clearly have a unique rapport. They've known each other for 35 years, as the host reminded his guest during Wednesday's interview.

When Trump ended a six-day boycott of all Fox News programs in September, he did so by appearing on O'Reilly's show. And he kept his commitment to appear on the program on Wednesday night, even after withdrawing from the cable channel's debate the following day.

What's more, the milkshake ledger shows that they socialize off the air. And their interview on Wednesday sounded more like an argument between old friends — an argument in which O'Reilly seemed to make little progress.

The point is that Trump can't be swayed, even by people he likes and respects. He does things his way. That's his whole political brand, right?

Ordinarily, a leading Republican presidential candidate would be expected to try to curry favor with Fox News, which is trusted by 88 percent of conservatives, according to a 2014 Pew Research Center study. But Trump seems to place a higher premium on his reputation as an outsider. He's already solidified his position as a Washington outsider and a Republican Party outsider.

"The last thing he can do is go outside of the entity that's known as a mouthpiece for conservative values," said Robert Thompson, director of the Bleier Center for Television and Popular Culture at Syracuse University. "If he's also outside of Fox News, baby, that's pretty far outside."

Pulling out of the debate carries some risk — a reality that Trump himself acknowledged when considering boycotts of previous debates. He could appear to be "chicken," as he put it when waffling in November over an upcoming CNN debate, which would undermine his strongman image.

But bolstering his outsider image is more important. He wants voters to know that nothing — not even milkshakes — can make Donald J. Trump do something he doesn't want to do.