The elections are now less than a month away, and rational observers are pretty well agreed that the Democrats are going to win. The big question is, by how much?

The Republicans have known all along that this was likely to be a Democratic year. The GOP has held the White House for eight years, and controlled Congress as well for six of those eight. In a two-party system like ours, the logic seems inescapable: "It's time for a change."

Only a really robust economic outlook could have altered this picture and given the Republicans a serious chance of victory. But instead, a government report has just announced that U.S. employers across a wide spectrum of industries eliminated 159,000 jobs in September -- the biggest monthly job decline in five years. The Republicans can read those tea leaves just as well as the Democrats.

The real question, therefore, is just how big the Democrat victory is likely to be. Will it be a relatively modest affair, or a thoroughgoing blowout?

Assuming Obama defeats McCain, and the Democrats extend their present control of both Houses of Congress, how big are their majorities likely to be? That question is of enormous importance, since the size of those majorities will determine the degree of dominance the Democrats will actually have. In the Senate in particular, the Republicans will be able to block Democratic initiatives if they have enough votes to wage a filibuster. Since a two-thirds vote of the Senate is needed to end a filibuster, the Democrats will need 67 votes. The Republicans can keep a filibuster going if they have only 34. But will they have 34?

At the moment, they have 49. That would seem to give them a fairly comfortable margin of 15 votes by which to maintain a filibuster. But if (as just about everybody expects) the GOP loses in November, that margin is almost sure to shrink -- perhaps dramatically. There have been times in American history when there were far less than 34 Republicans in the Senate. (In 1937, there were only 16 -- and, for good measure, just 88 Republican representatives in the House.)

So it is by no means out of the question that the Democrats in the next Congress might have a "filibuster-proof" majority in the Senate. With a correspondingly large majority in the House, the Democratic Party would have achieved something not far short of total dominance in American politics.

Still, realism compels us to recognize that such an overwhelming Democratic victory isn't very likely, even if they do remarkably well next month. Fifteen senatorships is an awful lot to pick up, especially when you already command a majority.

One must also remember that, even if the Democrats technically achieve a filibuster-proof Senate, it is always possible -- even likely -- that one or more members of that majority may not be inclined to vote, at the crucial moment, with his fellow Democrats. On the other hand, there may well be a Republican or two obstreperous enough to disrupt the coalition favoring a filibuster.

In short, obtaining enough control of the Senate to bar filibusters is going to be a very tricky business. That's the way the Senate's rules are designed, and the Senate obviously likes them that way. Every senator knows that the time may come when he personally will want to benefit from them.

In light of these considerations, voters who are planning to vote to put a Democrat in the White House next month may want to send a Republican to the Senate, if there is a Senate contest in his or her state. Omnipotence is not good for either party.