It is notoriously difficult for senators to become presidents, although the upper chamber’s 48-year drought will end in January.

What is less commonly remarked on is that it’s also difficult for presidential candidates to be senators. Or, to put it more precisely, it is difficult for presidential candidates who are senators to perform their duties as senators.

When you are focused on the open door of the White House, it isn’t easy to spare much time for the humdrum task of legislating.

This puts Senate Majority Leader Harry Reid (D-Nev.) in something of a fix. He is working with a slim 51-49 majority, counting the two Independents who generally vote with the Democrats, Sens. Joe Lieberman (Conn.) and Bernie Sanders (Vt.).

But two members of Reid’s conference, Sens. Barack Obama (Ill.) and Hillary Rodham Clinton (N.Y.), are fighting as hard as ever over the increasingly bitter Democratic presidential nomination.

That means they want to be on the campaign trail wooing voters rather than registering their own votes on this bill and that. (Indeed, it is often thought advisable for candidates to avoid votes so as to deny opponents campaign ammo.)

But if there is one piece of legislation that all senators need to vote on, it is the budget bill, and Reid needs Obama and Clinton in the chamber to push it through.

If neither of Maine’s centrist GOP senators, Olympia Snowe and Susan Collins, cross the aisle, as they sometimes do, and if Sen. John McCain (Ariz.) feels free to put in a day on Capitol Hill now that he has wrapped up the GOP nomination, then Reid will find himself outnumbered by Republicans voting no.

As The Hill reported Tuesday, the majority leader said on the floor, “It is my understanding all the presidentials will be here on Thursday,” but he went on to say, in words that must fall heavily on Clinton and Obama, “They’ll be here Friday if we don’t finish this bill on Thursday. They’ll be here on Saturday if we don’t finish the bill on Friday.”

Thursday … Friday … Saturday — for a campaign in the final stretch of the primary, that’s a chain of days that seems to extend to the crack of doom.

Reid has set deadlines before, only to break them. He scheduled a vote on the economic stimulus bill on the day before Super Tuesday and insisted that Clinton and Obama be there. Then he postponed the vote.

The budget is different, and the vote is unlikely to be postponed. Neither Obama nor Clinton can afford to be out on the trail if their rival is back in town doing his or her senatorial duty.