Clinton and Jimmy Carter have been fixtures at Democratic conventions since their presidencies ended, though Carter, whose presidency Democrats, like most Americans, don’t remember all that fondly, is usually trotted out nowhere near prime time. You have to go back all the way to Lyndon Johnson to find a Democratic ex-president who wasn’t included in convention proceedings: In 1972 (the only convention that occurred while Johnson was out of office and still alive), the debates over the Vietnam War, like the war itself, were still raging, and Johnson’s appearance would have proved hugely divisive at the convention that nominated George McGovern.

But what sins against Republicanism did today’s two Republican ex-presidents, George H.W. Bush and his son George W., commit? Both were mainstream Republicans of their times. Papa Bush presided over the death of Soviet Communism, and even if he wasn’t really responsible for its demise, you’d think that would be worth at least an appearance. But then, Papa Bush also raised taxes, which appears to have cast him into an ideological wasteland for today’s anti-tax Republicans.

As for the son, he promoted and signed into law massive tax cuts for the rich and did nothing to rein in the banks even as they did everything they could to magnify the risk they posed to themselves and everybody else. In other words, he followed Republican economic doctrine to the letter. He chose to wage a war of choice in Iraq, a war also sought by his party’s neo-conservatives. You might think that the fact that each of these policies ended in disaster would be reason enough for the Republicans not to invite W., but for the fact that these are still the policies that the party embraces (tax cuts for the rich, repeal of Dodd-Frank and attacking Obama for not doing more in Syria).

Bush’s banishing looks more like a case of ideological deviation than real-world catastrophe. He supported a path to legalization for illegal immigrants. He expanded Medicare to include a prescription drug benefit. (Obamacare, which the Republicans universally vow to repeal, provided more funding for that benefit.)

In other words, what’s wrong with the Bushes is the same thing that was wrong with Senators Richard Lugar and Robert Bennett, longtime party stalwarts whose routine bids for renomination were denied by Republican primary and caucus voters: they haven’t kept up with the party’s race to the right. The GOP base has banished the previous generation of Republican leaders for their lack of revolutionary zeal.

The tea partyization of the GOP has a lot in common with a sustained revolution, such as, to cite the paradigmatic example, that in France, where the Marats and Dantons, yesterday’s leaders, were cast aside for and by the even more zealous Robespierre and his ilk. The Republicans are Jacobins, and Jacobins don’t invite their old presidents back. When you’ve moved as far to the extremes as today’s GOP, even your own former leaders are the ancien regime.