Almost nobody seems to believe that Obamacare can be stopped under any realistic scenario given Democratic control of the Senate. And yet some House Republicans are willing to shut down the government—and possibly inflict a serious wound on their party—in pursuit of that chimerical goal.

What's going on here?

In fact, there is no one explanation. But here are my favorites in descending order.

Long-Term Politics. Hardline Republican strategists have recognized since the early 1990s that the GOP will be in political trouble if Democrats manage to deliver another major social benefit to ordinary Americans. If Obamacare succeeds, it will join Social Security and Medicare, among other programs, in the list of tangible ways that Democrats have improved the lot of voters—giving the party another advantage at election time for decades to come, just as the New Deal entitlement programs have bolstered Democrats for decades. Some Republicans are so desperate to stop this from happening that they are ready to throw a Hail Mary pass.

Short-Term Politics. President Obama is one of the least popular presidents of the modern era, as I have written here before, with polling numbers chronically below 50 percent. One reason is his healthcare law, which is his most unpopular major program—also with poll numbers below 50 percent. However, if the Affordable Care Act goes into effect and people start to like it—or at least see it's not so scary—the popularity of this program, along with Obama's overall popularity, is likely to rise. A more popular Democrat in the White House would be a more powerful Democrat, and no Republican wants to see that.

Big Ideological Donors. While your typical House Republican may not fully grasp the political stakes, the right-wing donor class surely does. And, as I wrote recently, groups like the Club for Growth and Heritage Action share much of the blame for the march toward a shutdown. Deep pocketed outside spenders have made it clear that members who don't support the attack on Obamacare may face generously funded primary challenges. The shutdown story must be seen as partly as a money-in-politics story.

The Illegitimacy Thing. The crazy campaign against Obamacare is hardly the first time that congressional Republicans have gone to the brink, and damaged their reputation, to thwart a Democratic president. House Republicans also pursued the impeachment of President Bill Clinton, even though polls said the public was against it and the effort seemed sure to die in the U.S. Senate. Back then, one common explanation for the anti-Clinton obsession was that hardline Republicans just can't accept the basic legitimacy of a Democrat in the White House. I think there's a lot to that, and we're seeing the same thing here—with Obamacare a proxy for Obama himself. After all, both Clinton and Obama aren't logical focal points of ideological hatred, with their moderate politics.

A Base Run Amuck. Finally, I definitely buy into the most common explanation of what's happening in Washington, which is that the hardline conservative base has become a Frankenstein that even Karl Rove can't stop. That base doesn't think practically about the GOP's approval ratings. Instead, it's focus is on ideological purity and it stands ready to punish members of Congress who don't do their absolute best to stop Obamacare.

While Washington is now moving fast toward a government shutdown, it remains to be seen whether the hardliners are willing to truly go over the brink in pursuit of a goal that is clearly unattainable.

If they do, and GOP poll numbers plunge with the 2014 election in sight, we'll have to add one more explanation to the list above: Stupidity.