The chances of a Republican majority in the Senate continue to slip away rapidly. Nate Silver’s forecast now shows the Democrats losing only one seat and retaining a 52-48 majority; the latest Rothenberg Political Report ratings show the same thing, with each party likely to lose one seat and Democrats holding four of the six pure toss-ups. Of course, the context was that this cycle was a huge Republican opportunity, coming six years after the Democratic landslide of 2006.

Tea party and other ideological Republicans have undermined that opportunity by nominating some weak candidates — this year’s prime examples are Todd Akin in Missouri and Richard Murdock in Indiana, who defeated a sure thing, incumbent Sen. Richard Lugar. That’s on top of Christine O’Donnell and Sharron Angle from 2010.

But it’s important not to overlook that each primary in which a tea party favorite defeats a mainstream conservative who would have been elected easily also affects recruitment by pushing good conservative candidates from running. So the real tea party story is just as much Florida and Pennsylvania, where Republicans failed to recruit a strong challenger to weak Democratic incumbents, as it is Missouri and Indiana. It’s even found in states such as North Dakota and even Wisconsin, where unpopular positions taken by national Republicans have rubbed off on candidates who must toe the tea party line to be nominated.

What’s really troubling for the GOP is that the effect will likely be even worse next time. There’s just no way that anyone can promise a good politician that she will make it to the general election on the Republican ticket. And don’t forget that most good general-election candidates are current office-holders, many of whom have to give up the safety of their current post if they choose to try for higher office.

So far, I’m not aware of any evidence that this effect is hurting Republicans farther down the ballot (although it could be; I haven’t seen evidence either way about whether, for example, the GOP had any unusual problems finding quality House candidates in this cycle). But if it keeps up, that’s the likely next step.

Of course, weak candidates can win anyway; however, a party that makes it harder to nominate good candidates for office is going to suffer in the long run, and may suffer severely.