WITH Jeb Bush committed and Hillary Clinton all but crowned, the easiest story to tell about 2016 is a tale of two dynasties, a Bush-Clinton grudge match, a fated collision between the first families of American politics.

But only one party is actively cooperating with that narrative. Clinton’s inevitability may inspire as much resignation as enthusiasm, but she’s currently so far ahead of her “rivals” that she looks like she could win the Democratic nomination with a William McKinleyesque front porch campaign . if her Georgetown mansion had one, that is.

On the Republican side, though, Bush’s alleged front-runner status doesn’t show up in any polls. He’s locking up money and talent, but his actual numbers are bobbing along in the teens, and his approval ratings are not the kind you associate with a man of destiny. He may win the nomination, but it will be a near-run thing, and nothing like the looming Democratic coronation.

Given the stereotypes about our parties, this is a notable reversal. The Republicans are supposed to be the party of primogeniture, the Democrats of fratricide. The now-ancient Will Rogers joke . “I belong to no organized political party; I’m a Democrat” . has been recycled for decades for a reason.

But since Barack Obama outlasted Hillary in 2008, the G.O.P. has fractured and squabbled and cannibalized itself, while the Democrats have become the lock-step party, their internal feuds sedate and their policy divisions mostly buried. Clinton’s unassailable position is specific to the former first lady, but it’s also part of a pattern where Democrats pick their candidates early . the winnowing of challengers to Kamala Harris in the California Senate primary is a case in point . and mostly avoid insurgencies in their primary campaigns.

To some extent this is normal: The stereotypes aside, both parties are more unified when they hold the White House and more fractious when they don’t. But the level of Democratic unity really is unusual by historical standards (Obama has faced much less intraparty opposition than did Bill Clinton or Jimmy Carter), and it’s particularly unusual given how poorly Democrats have fared outside of Obama’s presidential runs . losing the House in 2010 and the Senate last fall, and getting hammered in state politics.

At some point you would expect those defeats to have a fracturing effect, to inspire grass-roots uprisings or ideological splits. But notwithstanding the occasional surprise showing from a Zephyr Teachout or a Jesus Garcia, and notwithstanding the pining of the Warrenistas, the Democrats are poised to enter the lists in 2016 with the same “hang together” strategy they’ve taken throughout the Obama years.

One way to look at this strategy is that the party’s power brokers are effectively choosing the preservation of their impressive but ramshackle presidential-level majority, whose e pluribus unum qualities they don’t want to test with any kind of civil war over the kind of debates and experiments that might make the Democrats more competitive in reddish states and off-year elections.

The unvoiced assumption is that the coalition of 2008 and 2012 exists in a very precise equilibrium (more populism might alienate the liberal rich and upper-class professionals, a bigger tent on social issues might alienate the activists and depress turnout, etc.), and since it’s worked twice it’s worth sticking with again, even if it isn’t likely to take back Congress anytime soon.

At the national level, that choice makes a certain sense. The presidency increasingly bestrides our political system, liberals already have most of the welfare state they want and, as Obama’s imperial second term is demonstrating, a creative Caesar can use the bureaucracy to play offense on policy without having either the House or Senate on his side.

But the choice also has downsides, both in terms of what it concedes to Republicans (the entire South, to begin with) and in terms of the possibilities foreclosed, the escape hatches sealed off, when primaries and presidential nominations are predetermined.

I’ve written before that Hillary is the candidate most likely to hold a version of the Obama coalition together. But that doesn’t mean it will be pretty to watch. As we’ve been reminded by the revelations about all the foreign powers that donated to what is now known as the Bill, Hillary and Chelsea Clinton Foundation during Hillary’s tenure as secretary of state, she’s a celebrity on the surface and the very model of a postmodern machine politician underneath, with the ooze of corruption clinging to all the levers that she’ll need to pull to win.

For the sake of their existing presidential majority, the Democrats are lucky to have her. Where their integrity and ideals are concerned, maybe not so much.

And there may come a time, during the inevitable sleaze of a Clinton restoration if not sooner, when they may find themselves wishing they could just blow the whole thing up.