When I said, in December, that we were “probably at peak Republican,” I hadn’t considered yet the possibility that the GOP might capture the Senate, as now seems more like probable than possible. So let me say it here, now, and emphatically: Winning the Senate isn’t going to accomplish squat for the party; Obama needn’t overly fear the prospect; and — the kicker — success in the legislative branch is actually going to make it harder for Republicans to win the White House in 2016.

Picture it like this.

There are two chambers of Congress.

For Republicans, each is like a cement shoe.

Given the institutional structure of the Senate, wherein rural populations enjoy disproportionate representation, the vulnerability this year of Democratic senators in red states, plus the contemporary practice of gerrymandering House districts, it is now the case that an essentially regional party can win unified control of Congress. It can look like a national party without actually being one.

In practical terms, Republicans can win one chamber of Congress and keep another by running relentlessly on the repeal of Obamacare — but the same stance is probably a net loser nationally.

Similarly with immigration: congressional Republicans cannot take up the issue without dividing their ranks. And yet, as John Feehery has noted, the party’s inability to address immigration is a drag on the party nationally:

[I]f Republicans continue to express disgust with illegal immigration, if they continue to oppose comprehensive immigration reform, if they continue to show disrespect for folks who should be their natural political base, they will be a minority party at the national level, and they will never win back the White House.

There’s no easy way around this. Republicans are in a classic Hellerian catch-22: they’re crazy — and they’d be damn fools to behave any different. Their control of Congress depends, in many ultrasafe Republican districts and several deepest of deep red states, in part on fealty to conservative doctrine that will be problematic for the next GOP presidential contender.

There are green shoots, of course. On taxes, would-be reformers like Sen. Mike Lee and Rep. Dave Camp are incrementally edging away from the disaster that was the Romney fiscal agenda. It’s possible that the 2016 Republican nominee won’t be burdened with the albatross of campaigning on a tax reduction for the rich. That’s progress. The party passed a budget. That, too, is progress. But the seeming tranquility in Washington is simply the sound of two parties behaving well until a midterm election. If and when Republicans retake the Senate, the intraparty feud, now simmering, will begin to boil anew. The rightmost flank, flush with victory, will need to be appeased. And the ideological toxicity; the demographics of death; the lack of a viable national standard-bearer — these factors and others will conspire to elect the next President Clinton.

The fact is, we are two countries.

Republicans dominate the smaller one.

The consolation prize, awarded in the off years, is Congress.