WHEN Donald Trump began roiling the calm but crowded waters of the Republican presidential race with slanderous comments about Mexican immigrants, it was hard not to wonder what the rentier reality-TV star was up to. When someone runs for president, the safest inference is that he or she wants to be president. Even if it's a longshot, the attention is nice, and something good might come from having raised one's public profile. So if you have the money, why not run? Mr Trump's lust for attention, combined with his fortune, seemed to be all the explanation needed. "Do I look like I have a plan?" says the Joker in "The Dark Knight". "I'm a dog chasing cars. I don't know what I'd do if I caught it". Mr Trump's havoc-spreading run seemed to share this improvisational spirit. But now that he's leading the Republican field in national polls, it seems that Mr Trump may have already caught the car, and always knew what he'd do with it: sell it for a profit.

When asked if he would pursue a third-party candidacy should he fail to secure the GOP nomination, Mr Trump was coy: "so many people want me to, if I don’t win,” he said. "I’ll have to see how I’m being treated by the Republicans,” Mr Trump explained, with just a hint of blackmail. “If they’re not fair, that would be a factor.”

These are the words of a negotiator looking to cut a deal, and Mr Trump has put himself in a fine position to get one. The latest ABC News/Washington Post poll not only has Mr Trump leading the Republican pack, but also shows that a third-party run by Mr Trump could very well ruin the party’s chances of winning back the White House. The poll has Hillary Clinton, who retains a commanding lead in the Democratic race, with a six-point lead in a hypothetical contest against, Jeb Bush, a former Florida governor and the likeliest GOP-establishment candidate. That gap would probably narrow into margin-of-error territory should the Republican electorate, whose loyalties are now fragmented, eventually coalesce behind Mr Bush. Which is to say, an election would probably be very close, but not if Mr Trump makes a third-party bid.

What explains El Donald's rise? In a hypothetical three-way race, Mr Trump would steal enough votes from Mr Bush to put Mrs Clinton ahead by 16 points, according to the ABC News/Washington Post poll. The Donald’s conservative vote-siphoning effect would be less dramatic, no doubt, should Republicans settle on a candidate more appealing to the party's most conservative members. (There is method in Ted Cruz's refusal to condemn Mr Trump's madness.) Nevertheless, it appears that Mr Trump could very well hand the election to Mrs Clinton, even if he manages to draw only one or two percent of the voters who would have voted for the Republican candidate in a two-way race. And it’s a prospect he has yet to rule out. It's tempting for establishment Republicans to write off Mr Trump as a flash in the pan, this election season's Herman Cain. There are signs that Mr Trump is peaking. In three key swing states, voters with a dim view of Mr Trump outnumber those who like him by a 2-to-1 margin. Still, Mr Trump is famous, which ought to be enough to keep him from sliding into oblivion. And he is willing and able to fund his own campaign. That makes the threat of a third-party candidacy something Republicans cannot afford to ignore. And it’s a threat Mr Trump has made explicit. So what, Republicans may now be wondering, does the man mean when he demands “fair” treatment? A cabinet appointment is out of the question. Is he angling for an ambassadorship to Monaco (or somewhere else suitably glamorous and otherwise diplomatically inessential)? It’s unclear. But what we do know is that Mr Trump is in the real-estate business, an industry rife with red tape, in which a little political leverage can be worth a fortune. In his bestseller, "Trump: the art of the deal", Mr Trump crows about a property-tax abatement he negotiated with the New York City government in the 1970s that saved him "tens of millions of dollars". Then in 1994 Mr Trump asked the city of Bridgeport, Connecticut, to condemn land already occupied by five businesses, and then sell him the parcels so that he could build a "world class" amusement park and seaport. Noting the difficulty of acquiring the property and the required development permits, Mr Trump said, "That's a political process. And I've probably been in more political fights than anyone my age". In 1996 Mr Trump tried and failed to push an elderly widow out of her house using Atlantic City's eminent domain powers, so that he could build a casino on her land. More recently, Mr Trump tried and failed to block the development of a casino in the Catskills, which he feared would cut into the profits of his New Jersey gambling operations. He had considered appealing to the Department of the Interior to stop the Native American-run Catskills casino, but gave up in the teeth of then-governor Eliot Spitzer's support for the project. "At some point there’s going to be competition," Mr Trump eventually conceded. But he hadn't in the previous ten years, during which time he successfully delayed the development through lobbying and litigation. Given his history of remunerative favour-seeking from government, it's doubtful he would have ever conceded if he thought he could get the federal government to put a stop to it.

You'd have to be astoundingly brazen to run for president, churning up toxic xenophobic sentiments, just to get the political leverage to win a huge tax break, or to build a casino or to stop somebody else's casino. But Mr Trump is neither a meek nor public-spirited man. And, astonishingly enough, he may have actually succeeded in putting the Republican Party in a corner.

If cutting a sweet deal is what Mr Trump was aiming to do all along, we might have to admit that he is more than the attention-seeking buffoon he appears to be. It may be that he is an attention-seeking, buffoonish genius. In any case, Mr Trump has floated the possibility that he may try to wreck the Republican Party's presidential chances unless it coughs up a little "fair" treatment, whatever that means. If the GOP doesn't think it can neutralise Mr Trump's threat of a third-party run by utterly demolishing his reputation, then they're going to have to consider a little fairness. Not a bad month's work for Mr Trump.