Washington ? When Virginia Sen. George Allen conceded to Jim Webb last week, giving the Democrats their 51st seat in the Senate, the responsibility for the work of the legislative branch shifted definitively – and a potential weak point for the new majority was exposed.

Hours earlier, it became clear that the Democrats had won a majority in the House of almost the same size that Republicans had enjoyed for the past two years – a working margin large enough to support a fairly ambitious agenda. Nancy Pelosi, who will be the speaker of the House, has the personal strength and political skills to hold that majority together, at least as well as Dennis Hastert did in his time at the helm. House rules favor majority control, so Pelosi can be generous toward the minority without jeopardizing her chances of success.

The 51-49 Senate is a very different proposition. As Republicans learned to their chagrin in the last few years, it takes 60 votes to accomplish almost anything controversial or substantive in the Senate – the number needed to bring debate to a close and force an up-or-down vote. That means persuading or pressuring at least nine Republicans to go along. Otherwise, Sen. Mitch McConnell, the crafty Kentuckian elected Wednesday as the Republican minority leader, will have veto power over legislation – before it goes to the White House, where President Bush will wield a veto pen of his own.

That reality puts an enormous strain on Harry Reid, the Nevadan who has struggled these past two years to master the less arduous duties of Democratic minority leader. The risk for Democrats is that Reid may not be up to the challenge.

We saw how weakness in the majority leader could hurt a party in the recent example of Sen. Bill Frist. More than once, Frist was unable to hold his own troops together on big issues or to thwart Reid and the Democrats when they built roadblocks to administration bills. Now the situation is reversed and it is Harry Reid who will be tested.

The modern Senate is a haven for freelancers, for senators who play to national constituencies outside the Capitol and whose energies are focused on their personal ambitions. That is especially true during the presidential campaign season, which unfortunately is already upon us.

The Senate is chockablock with presidential wannabes. On the Democratic side, they include the leading contender, Hillary Rodham Clinton, of New York, as well as Evan Bayh, of Indiana; Joe Biden, of Delaware; Chris Dodd, of Connecticut; Barack Obama, of Illinois; and – in his own mind, at least – John Kerry, of Massachusetts. On the Republican side, the potential leading candidate, John McCain, of Arizona, has many friends, among them such influential and ambitious senators as Chuck Hagel, of Nebraska; Trent Lott, of Mississippi; Lindsey Graham, of South Carolina; and John Sununu, of New Hampshire. Another possible candidate is Sen. Sam Brownback, of Kansas, who represents the religious right.

In addition to those egos and ambitions, Reid will have to contend with a few real mavericks and independent spirits in his caucus. Sen. Joe Lieberman, of Connecticut, elected last week as an independent Democrat after losing his party’s primary to Ned Lamont, has reiterated his intention to follow his own path rather than accept party discipline. He has staked out a position on Iraq – opposing any timetable for withdrawal – that is at odds with the prevailing inclination of the new majority.

And Webb, the man who gave the Democrats that majority, will now give their leadership anxious moments. A newcomer to elective politics and resentful of the conventions and demands of the political game, Webb is also a newcomer to the Democratic Party. His previous government experience was as Navy secretary in the Reagan administration. His campaign showed him to be a populist on economic issues, a sharp critic of U.S. involvement in Iraq – but often unpredictable or uncertain about other questions. Most of all, he resisted being managed or directed, so Reid will have to handle him with kid gloves.

As minority leader, Reid was remarkably effective in keeping the Democratic caucus united, but was far less successful as a public spokesman for his party. His partisan comments were often too sharp; his television appearances, less than commanding. In his new role, he will be far more exposed, and his flaws more conspicuous.

With a House majority and nominal control of the Senate, the Democrats will be expected to deliver on their promise to bring the Iraq war to some kind of conclusion and redirect domestic policy to the benefit of the middle and working classes.

That is a large order for the Senate Democrats and their tiny majority – and their leader, Harry Reid.