I switched enrollment from Republican to Democrat when the GOP went off the rails, to the dark side; I wanted to use my primary vote to keep at least one party on the rails. And I was glad when actor Cynthia Nixon (“Sex and the City”) challenged vile Governor Andrew Cuomo — even though from the left.

The other night Nixon was on The Daily Show.

At the start, the question was whether she’s really out to become governor, rather than just making a point.

Someone serious about the governorship might focus on the state’s real problems — like its weak economy beset by high taxes, a lousy business climate, and crumbling infrastructure. Instead, Nixon was all pet “progressive” memes. Such as inequality, $15 minimum wage, and “women’s issues.” Repeatedly chanting the word “progressive” as a talisman.

Cuomo himself has been doing the same, leaving little space to squeeze into on his left. But Nixon is trying, calling Cuomo insufficiently progressive. Like on the $15 minimum wage, being introduced gradually rather than immediately.

This kind of left-wing purity trial will kill the Democratic party for 2020. There’s already a big debate within the party over whether to countenance anyone not 100% for abortion. Are these people out of their minds?

Republicans have left vacant a vast territory in the American political center. But instead of moving briskly to seize that territory, Democrats like Nixon (and Cuomo) are fleeing it too.

Meantime, Andrew Cuomo is a very strong candidate, still basking in his father’s glow, with the party organization locked up, and a huge campaign war chest. How will Nixon compete with that?

I’m reminded how last year, a guy sought me out who wanted to run for Albany Mayor on the Republican line. The Democratic incumbent, Kathy Sheehan, had done reasonably well, and, mind you, the city is something like ten-to-one Democratic. No Republican had been elected to anything in almost a century. So this guy starts telling me all his nifty ideas for how Albany could be spruced up. One was something about bike paths.

Finally I stopped him and said, “All very nice. But is Sheehan going to lose to a Republican on the issue of bike paths?“

The point being that to defeat a powerful incumbent you need a powerful issue. And Cynthia Nixon actually has one: public corruption. Andrew Cuomo is drenched in slime. His top honcho, Percoco (his “third brother”) was convicted of taking bribes, peddling his influence with the governor, and mis-using his position right under Cuomo’s nose. While another top Cuomo guy, Percoco’s partner in crime, pled guilty and testified against him while being revealed as a comic book sleazeball.

State government corruption was already so odoriferous in 2013 that Cuomo convened a special blue-ribbon “Moreland Act” commission to investigate and take action. Then he pulled the plug, disbanding the commission — when it started looking at the governor’s office. Eeewww. To her credit, Cynthia Nixon has pledged a new Moreland Act commission.

But did she mention any of this on The Daily Show? Not a word. The word “corruption” did not pass her lips. Instead she hammered the word “progressive” and stuff like “women’s issues.”

At the very end, Cuomo’s $31 million campaign kitty did come up. As something Nixon would have to overcome. I literally shouted at the TV: “Tell us how he got the money!”

She did not. Did not mention he got it by selling favors to contributors, paid for by taxpayers. Like the real estate developer who gave Cuomo’s campaign big bucks, and then was repaid many times over with a huge unnecessary public subsidy for a project which was already underway.

On all this Nixon was silent. She has one issue, and one issue only, that could conceivably elect her, and she ignored it — so wrapped up was she in her “progressive” shtick. Corruption isn’t an issue with a satisfying ideological thwack.

Last time around, Cuomo was similarly challenged from the left by Zephyr Teachout — who got a third of the vote. A lot of her votes, ironically, did not come from ideological “progressives” but rather upstaters angry over Cuomo’s gun control legislation. That’s why Teachout carried around 30 rural counties (while being crushed downstate).

Nixon will get all the votes Teachout got. And not one vote more.

Correction: one vote more. Mine.