Among superdelegates who have endorsed a presidential candidate in recent weeks, three out of every four have backed Sen. Barack Obama (D-Ill.) rather than Sen. Hillary Rodham Clinton (D-N.Y.).

That may be just because they think he is the better candidate, but if it were that simple, they would probably have decided before now; we’re into June and the 18th month of official campaigning.

Many have come to their conclusion, rather, because they think Obama is now the inevitable nominee and, like most people — indeed, more than most people — politicians try to make sure they are on the winning side.

Thus the momentum builds. As more superdelegates rally to Obama, his nomination becomes more likely, and as it becomes more likely, more superdelegates rush to join him. Even Sen. Robert Byrd (W.Va.) and Rep. Alan Mollohan (W.Va.), who represent a state in which Clinton beat Obama by a massive margin, opted for Obama last month.

The gathering avalanche toward Obama looks likely to accelerate this week. The last two Democratic primaries, in South Dakota and Montana, are being held Tuesday. Clinton has stayed in the race, arguing, reasonably enough, that she should not be obliged to withdraw before voters get the chance to make their choice — no matter how inconvenient that is to the party hierarchy.

But with no more primaries from tomorrow onward, and with Clinton not a handful of delegates behind Obama, but hundreds, the pressure on her to concede defeat will intensify exponentially. Her hope of claiming the greater popular vote involves a calculation that is, if not implausible, certainly thoroughly debatable.

And superdelegates do not seem persuaded by her other contention — this one surely stirs fugitive fears in their breasts — that she is more popular than her rival in swing states without which it will be difficult to get a Democrat to the White House.

As The Hill reports Tuesday, many undecided lawmakers say they will definitely make their endorsements this week. Still more say they are likely to do so. And some who intend to hold out yet longer are doing so, reportedly, not because they wish to deny Obama their backing but because they wish to give Clinton a grace period in which to make an admirable and unhurried exit.

Both House Speaker Nancy Pelosi (D-Calif.) and Senate Majority Leader Harry Reid (D-Nev.) say they expect the nomination fight to end this week, or within a very short time thereafter. They do not expect a convention floor fight. Superdelegates appear to be signaling a similar conviction, and that this long, tenacious battle of attrition really is now almost over.

And then, of course, the bigger battle begins.