For Republicans, it’s conjured as a doomsday scenario: On Election Day, Barack Obama wins the White House and Democrats expand their majorities in both the Senate and the House. With the executive and legislative branches under a unified party banner, get ready for a raft of liberal legislation that will irrevocably push the country to the left.

Conservatives may worry about living in this waking nightmare come January, but there are several good reasons why they shouldn’t despair.

For one, while the Democrats have been ascendant in recent years, they have entered into a Faustian bargain. They’re gaining seats, but in doing so they’ve made compromises which have resulted in the creation of an ungovernable majority.

Over at The Next Right, the blog home of some of the biggest political geeks in all of GOPdom, Sean Oxendine ran the stats. The American Conservative Union (ACU) ranks all members of Congress according to how conservative their voting records are on a scale of one to 100. Here’s what happens when you rank the Democrats in the House of Representatives according to how conservative they are:

Its most conservative member is Tom Barrow. HIS ACU is lifetime is 47, and has gone as high as 76 last year (well into the Republican caucus). 15 in is John Tanner, who still has a lifetime ACU of 43. Go in 45, and you find Congressman Costello, whose lifetime ACU is 34. Go another 20 in, to 65, and you’re at Congressman Ortiz, whose lifetime ACU is 33. You’re now a little less than 1/3 of the way into the Democratic caucus, and you have a member who votes with the ACU about 1/3 of the time.

“In other words,” Oxendine sums up, “well into the Democratic caucus, you have members with some pretty conservative instincts.” By contrast, if you order the Republican caucus by most liberal, the most liberal House Republican has an ACU rating of 60, and ACU rankings from there rise steeply. The fact is that the Republican leadership presides over a much narrower ideological spectrum than the Democratic leadership. Thus, Republicans are more likely to agree with each other, are easier to predict, and easier to rally to a cause.

The consequences of this played out pretty dramatically with the financial bailout bill last week. When it failed in the House, two thirds of Republican congressmen voted against it and thus were given the lion’s share of the blame for its failure.

But when you scratch the surface beyond looking at the raw vote totals, the bill’s failure to pass begins to look a lot more like a total breakdown of the Democratic leadership. Given that Republicans are ideologically predisposed against massive socialist interventions in financial markets, their vote was hardly a surprise. However, 95 Democrats also voted against a bill that failed by 12 votes. Pelosi couldn’t rustle up an extra 12 votes out of 95?

Some of this is naturally attributable to Pelosi’s general incompetence — when it comes to wrangling up votes, Pelosi is no Tom Delay. But the reality is that the Democratic caucus is so conservative, Pelosi wasn’t in a position to tell many Democrats, especially Southern Dems and those inhabiting recently captured Republican seats in conservative districts, that they had to vote for it.

Aside from this ideological chasm making it difficult for the Democrats to govern, it also makes the Republicans an unusually effective minority. After the Senate took up the bailout bill, Republican Minority Leader Mitch McConnell was able to add a few key changes to the bill and got 33 out of 49 Republican senators to vote in favor — allowing the bill to pass in the Senate by almost the same percentage that it failed in the House. With McConnell’s changes, influential Republican congressmen (such as John Shadegg) who opposed the bill initially decided to throw their support behind it the second time around, and brought other Republican votes along with them. More Republican votes for the bill also meant additional cover for conservative Dems to vote for it. So it is hard not to view its passage as the result of a nimble and effective Republican minority.

And yet, Democrats continue to pursue a strategy that’s about expanding their majority uber alles, governance be darned. One notable vote against the bill was Mark Udall from the second district of Colorado. The second district is centered in Boulder, Colo. — a college town that’s among the most liberal places in America. True to form, representing Boulder Udall has an 8 percent ACU rating.

But he’s running for Senate this year, and in one of the most shamelessly craven transformations in recent political history, Udall is attempting to campaign as a moderate-to-conservative Democrat to drum up his statewide appeal in purple Colorado. He’s a dogmatic environmentalist whose wife is former legal counsel to the Sierra Club — and suddenly he favors domestic oil drilling. And the Boulder liberal who co-sponsored legislation with Dennis Kucinich to spend $8 billion to create a cabinet-level Department of Peace is suddenly a fiscal hawk when it comes to the bailout. So Pelosi excused, rather than pressured Udall for his vote on the bailout. Even if Udall does win his election (he’s narrowly leading), how does he expect to get re-elected unless he suddenly starts consistently voting far more conservatively than his previous voting record would predict? Having Udall in the Senate is obviously a net plus for Democrats, but if he wins, he’ll have done it by painting himself into a red political corner.

In fact, the Senate and House are full of Democrats who are either legitimately conservative in ways that cut against their party, or who represented themselves as such to get elected. The silver lining here for Republicans is that the Democrats’ inability to govern suggests that the electoral shift in favor of Democrats doesn’t indicate a major swing in voter attitudes and values. It’s still a center-right country and there should be plenty of opportunities to reclaim seats in Congress from Democrats in conservative districts, especially from those who won office by representing themselves as more moderate or conservative then their party.

So now, how would President Obama get through, say, a sweeping leftward expansion of government-provided health care using a legislative majority that includes folks who vote with Republicans three-quarters of the time? Under Pelosi, Congress has a 9-percent approval rating and has trouble getting basic appropriations bills passed. For the foreseeable future, any ambitious new spending or structural changes that Obama might have planned on getting through congress — especially if the treasury gets emptied of $700-plus billion in an attempt to solve the credit crisis — would seem D.O.A. even with an expanded Democratic majority.

It might be an exaggeration to say that in the current congressional climate Obama would be better off with Republican opposition in the legislative branch. But if Obama can’t get anything done in a Democratic congress, he’ll suffer politically, giving Republicans another opening to exploit.

Whether Republicans are canny enough to realize and exploit this structural weakness in the Democrats’ congressional majority is anybody’s guess, but for now it’s enough for them to remain hopeful no matter what happens in November.

– Mark Hemingway is an NRO staff reporter.