It’s looking increasingly likely that Democrats will hold the Senate but not the House, although upsets in either chamber remain possible.

In recent days, I’ve heard a lot of speculation about why this is so. Common answers are that the personalities of the candidates matter more in the Senate (this is possibly true: partisan crossover is about 15 percent in Senate races, versus about 10 percent in the House, which might indicate that voters scrutinize the candidates more carefully), that the G.O.P. has a comparatively weaker set of candidates in the Senate (I think this is actually much more debatable than it seems), or some combination thereof.

These explanations are really missing the boat. Sometimes, the reason for something is so obvious that it becomes easy to miss; this may be one of those times.

The reason that Democrats are most likely to hold the Senate but not the House — the necessary and sufficient reason — is because only one-third of the Senate is up for re-election every two years. If the whole Senate were up for re-election next week, Democrats would lose it and lose it badly.

Take a look at our Senate forecast map. There’s a lot of red there. Part of that, yes, is because Republicans tend to do better in the middle of the country where the states are physically larger — but that kind of misses the point.

Right now, among the 37 Senate elections, we have Republicans favored in 25, Democrats favored in 11, and one (Colorado) that’s too close to call. If Democrats have a relatively good election night, they will win about one-third of the available Senate races. And if anything, the states that are voting for Senate this year are slightly blue-leaning. If the entire Senate were up for re-election in this political climate, the Republicans would be favored to earn a filibuster-proof majority, and might even earn a veto-proof majority! Fortunately for Democrats, that’s not how the system works. (Maybe some of our readers could go though the list of 63 senators that are not up for re-election and guess which ones they’d expect to lose if they were. It could be kind of fun.)

By comparison, in the House, where everyone is up for re-election every two years, Republicans appear most likely to win something like 53 percent of available seats. The fraction could conceivably approach 60 percent if they have a really terrific night, or it could be a bit below 50 if the Democrats overperform their polls and hold the House. But the Republicans almost without doubt will win a higher fraction of the available Senate seats (and probably also the available governors’ seats, although that could be a lot closer) than they will in the House.

Now, you also can’t go to the other extreme and suggest that Democrats are actually underachieving in the Senate, because Senate seats — which represent entire states — are a little bit more heterogeneous than House seats, which are often highly gerrymandered. The Senate seat that is in the 75th percentile on the spectrum from red to blue is something like Maine or Washington or New Jersey, which is about 5 percentage points more Democratic than the country as a whole. The 75th percentile House seat is something like Michigan’s Fifth Congressional District or California’s 39th or Colorado’s Second, which is about 10 or 11 points more Democratic. The typical Senate seat is a lot ‘swingier’ than the typical House district.

If you adjusted for this, I’d expect you’d find that the Democrats are performing about on par in each chamber, losing most (although not all) swing districts, some (although certainly not most) blue-leaning districts, and almost all (although not absolutely all) red-leaning districts.

Now, this is not to suggest that Republicans don’t have some “candidate quality” problems in the Senate. They do. I think you can argue, for instance, that Generic Republican should beat Generic Democrat in a state like Colorado in a political climate like this one, when in fact the race is about tied there. Generic Republican would probably also be favored over Generic Democrat in Nevada, and would almost certainly be favored over the nongeneric (and very unpopular) Harry Reid, but the race is tight there, too. Even in Kentucky, which Rand Paul seems likely to win, the race would probably not be within 5 or 6 points in an environment like this one had the Republicans picked a more orthodox nominee.

But the Democrats also have some candidate quality problems, even if their candidates do not tend to commit the made-for-cable-TV-news gaffes that someone like a Rand Paul or a Sharron Angle does. My hunch is that Shelley Berkely would probably be crushing Ms. Angle in Nevada were she on the ballot in place of Mr. Reid; Lisa Madigan would probably have a clear lead over Mark Kirk in Illinois; there are even states like Arizona — where John McCain’s approval ratings are actually quite poor — in which an absolutely top-tier Democratic nominee might have made a competitive race. And meanwhile, the Republicans have some strong candidates, including both establishment choices like Rob Portman in Ohio and John Hoeven in North Dakota, and antiestablishment ones like Marco Rubio in Florida (a Tea Partier), and probably even Ron Johnson in Wisconsin (another Tea Partier), who has run a really smart campaign, although he’s not quite out of the woods yet against the incumbent, Russ Feingold.