The easy-breezy days of running circles around a divided GOP field have come to an abrupt end. Photo: Brendan Smialowski/AFP/Getty Images

Yes, it must be fun and ego-gratifying to be Donald J. Trump right now. He’s won the GOP presidential nominating contest without the complex nightmare of a contested convention, and without significantly depleting his personal fortune (so far as we know). The Republican and movement-conservative Establishments are grudgingly but steadily moving into his camp. When he comes to Washington for meetings with his vanquished intra-party foes, it’s with the air of a barbarian chieftain visiting Rome after the Sack. His opinion-leader and journalistic detractors are in a state of cowed confusion over their failure to accurately assess his political strength. Pollsters are entering his presence with surveys showing he could indeed beat Hillary Clinton (he needs only one every once in a while to support his vainglorious talking points). And he still has his formal coronation as the most unlikely GOP nominee since Wendell Willkie ahead of him before the general-election campaign gets fully underway. For the Donald, life is good, right?

Maybe so, but not for long.

Throughout the pre-primary and primary phases of the GOP presidential-nominating process, Trump had a bunch of advantages he will soon lose. He was a novice pol who was regularly defying expectations amidst almost universal predictions of failure. He was the dominant media object in a very crowded field of opponents. He had the strategic flexibility associated with doing relatively well in every region of the country and among every major category of Republican voter. He was independent of any sizable bloc of endorsers, donors, or surrogates, operating from his own tight-knit personal army. He was functioning within a Republican Party dominated by the older white voters that were his principal base, and where the minority voters he so deeply offends were rare and insignificant presences.

Now he is about to become the Titular Head of the Republican Party, with presumed responsibility for a big, divided and (at the moment) fearful coalition of down-ballot candidates and allied constituency and interest groups. Even if he minimizes the value of party support, he’ll have to deal with constant advice and admonitions from party officials, many of whom not-so-secretly would prefer that he lose and leave them to inherit the GOP. He’s already beginning to hustle money to finance his campaign.

Given the binary nature of general elections, he can no longer count on the kind of huge margins in media coverage he enjoyed when it was 16 Lilliputians trying to overcome the orange-haired Gulliver. For that matter, in Hillary Clinton he will finally face an opponent as well known as he is. He will not be able to run a national campaign that divides and conquers a scattered and regionally dependent opposition. He’ll be fighting Clinton one-on-one in the same fixed set of battlefield states. Instead of dealing with an electorate where he can find support all across demographic groups, Trump will be beginning in a deep, deep hole with African-Americans, Latinos, and professional women, with sure support only from groups like non-college-educated conservative white men, which any Republican can and must carry by huge margins.

At key moments in the campaign like the debates, Trump will no longer be addressing an audience that inherently hates “political correctness” and thus has a high tolerance for borderline racist and sexist rhetoric and insult-comedy. And Clinton and her allies will be able deploy their massive oppo-research files on Trump in a consistent, relentless manner very much unlike the occasional, clumsy, and halfhearted Trump-bashing undertaken by his primary opponents and the mainstream media. After all, it’s not like Democrats need to treat him with kid gloves because they’ll need to appeal to his core supporters down the road.

Most of all, if the worm ever turns on Donald Trump’s immensely lucky 2016 campaign, it’s likely to turn fast and hard. Much of his party will abandon him in a heartbeat if that’s the best way to preserve Republican control of Congress and state governments. The media folk he despises and seeks to threaten and intimidate will be unforgiving if he begins to stumble. It could get very ugly very fast.

Perhaps Trump will be luckier and more skillful than I suspect in the very different context of a general election. But anyway you cut it, he’s going to have a lot of white-knuckle moments from here on out. And it just isn’t going to be as much fun.