In Thursday night’s debate, Donald Trump got an avalanche of boos for declaring that he would not pledge to abide by the decision of Republican primary voters and support the eventual nominee if he did not prevail. Trump explicitly left the option open of running a third-party campaign if he did not “respect” the eventual nominee, a position that got some cheers at the end, followed by a louder chorus of booing from the live audience at the Fox News Channel debate:

Less than four days later, Trump may have changed his mind. ABC’s Jon Karl reports that Trump has begun mulling over a public pledge against a third-party run:

Donald Trump may soon do what Republican leaders have been asking the billionaire candidate to do — pledge not to run as an independent candidate for president, a senior Trump adviser told ABC News.

Trump refused to take that pledge at the start of Thursday’s GOP debate but is now seriously considering promising not to run as an independent if he does not win the Republican nomination, the adviser said.

What changed? Karl believes that Trump and his team may be worried that the RNC will lock Trump out of the debates, especially in the aftermath of Trump’s personal attacks on Megyn Kelly all day after the last debate, but mainly because of this answer on a third-party run:

Republican National Committee Chairman Reince Priebus has directly called on all candidates to pledge not to make a third-party run. The Republican Party has leverage here: senior GOP officials say it is possible that future debates could be limited to candidates who have pledged support for the party — a move that could leave Trump out of debates if he is still leaving open the possibility of running as an independent.

Karl also notes that the GOP can withhold voter data from the primary campaigns. The RNC has worked very hard on improving its data-collection infrastructure, and that data would be invaluable to primary campaigns. It would cost a campaign a fortune to start from scratch on that kind of research, and even then, there would be almost no time left to accomplish it — even for someone of Trump’s wealth.

This looks a bit like someone at the RNC — Reince Preibus, presumably — has let it known that he’s prepared to call Trump’s bluff on a third-party run. It takes a tremendous amount of resources to pull that off, and Trump hasn’t even begun to put together a traditional primary organization, let alone something on that scale. Republicans and Democrats have state organizations in place to get their nominees on ballots, but an independent Trump would have to do that by himself in all 50 states.

Ross Perot started working on that early on in the 1992 cycle, in part by spending millions of his own money and in part by building coalitions with existing minor parties in some states to build what became the Reform Party. Thus far, there has been little evidence that Trump plans to invest much money or effort into his campaign. He spent $1.8 million in the second quarter, all but $93,000 of it his own money, and none of it on significant organization. He’s going in cheap thus far, but if Preibus cuts him out of the debates and access to the data, he’d either have to start spending and raising money prodigiously to compete or bail out.

At this point, it’s easier to take the pledge. The question will be whether anyone can call Trump’s bluff at that point. If he pledges no third-party run, the RNC will have to give him the same access as the other campaigns, and later Trump can come up with a rationalization for breaking the pledge if he wants. But that threat will force Trump to either get serious or get out.