Then-White House chief strategist Stephen K. Bannon speaks at the Conservative Political Action Conference in National Harbor, Md, on Feb. 23. (Joshua Roberts/Reuters)

Stephen K. Bannon knows virtually nothing about American politics or economics. If he did, he wouldn’t say inane things such as, “Economic nationalism is what this country was built on.” Actually, it was built by wave after wave of immigrants, and trade protectionism helped usher in calamities such as the Great Depression. That said, he knows something about politics when he says that President Trump would not be facing a special prosecutor if he had not fired FBI Director James B. Comey. As Bannon put it, it was the worst mistake in “modern political history.”

The second worst mistake would be what Bannon is instigating — an assault on GOP incumbents, especially senators. The Republicans have a narrow two-vote advantage, with Sen. Dean Heller (R-Nev.) and Sen. Jeff Flake (R-Ariz.) the most vulnerable — so Bannon is going to make them more vulnerable by running wacky challengers not only in those states but in others previously not thought to be in play. Um, isn’t that the Democrats’ dream come true?

Yup. Running alt-right challengers does five things, at least, to undermine Republicans’ effort to hold the Senate.

First, if one of them (e.g. Kelli Ward in Arizona) should dislodge an incumbent in the primary, Democrats will get the chance to run against an extremist and a pet poodle of the president. The president is very unpopular, you might have noticed, so a Trump-Bannon stamp of approval may doom a handpicked candidate.

Second, running an alt-right incumbent will pull incumbents to the right on trade, immigration and race — making it harder to win in the general election if the incumbent does survive a primary challenge. If an incumbent does emerge, bloodied, with depleted resources and comments preserved on video that make him or her sound like a Bannon acolyte, Democrats will be that much closer to picking up the seat.

Third, the prospect of running against a nutty challenger may drive incumbents into retirement. Take, for example, Sen. Bob Corker (R-Tenn.). Politico reports:

Corker, who chairs the influential Senate Foreign Relations Committee, is still considering whether to run for a third term next year. His retirement would cause a reshuffling on top Senate committees and potentially tighten the race for control of the Senate, which heavily favors Republicans.

“I think everyone in the Volunteer State knows, as they did in 2012, that running for reelection has never been an automatic for me. While we are in a strong position, I am still contemplating the future and will make a decision at the appropriate time,” Corker said Monday.

Few thought a Tennessee Senate seat might be in play, but nominating an alt-right ignoramus in a state used to moderate Republicans would be unwise, to put it mildly. (Democrats had better start scrambling to find decent challengers in all races, given the chance they could be running against a Bannon stooge.)

Fourth, what’s Trump going to do? Campaign for or against incumbents? Donors and primary voters may get mighty confused trying to figure out whether Trump is on the side of the incumbent or on the side of a challenger with little hope of winning.

Fifth, seeing challengers coming down the pike will increase the “every man for himself” attitude that already has permeated Congress. Go along with a president on a tough vote when the president is going to stab you in the back in a primary? If you thought dealmaking was tough before, wait until Republicans sense that Trump is scheming to unseat them.

Democrats surely see a helping hand here. Having both the House and Senate majorities would have been unthinkable after the election wipeout in November. Now it’s not impossible to imagine. And with the majority comes the power not only to pass hostile legislation but to conduct oversight, subpoena witnesses and documents, authorize an independent commission on Russia, and, of course, impeach and hold a trial for a president. Are we sure Bannon’s not a closet Democrat?