WASHINGTON  The possibility of a victory by Senator Barack Obama combined with significant Congressional gains by his party could give Democrats extraordinary muscle to pursue an ambitious agenda on health care, taxes, union rights, energy and national security.

Democrats, who are within reach of the 60 votes needed to break a filibuster in the Senate, would also face high expectations, especially from the party’s more liberal quarters, that could be difficult to meet even with enhanced numbers in the Senate as well as the House. And they would be at risk of overreaching, a tendency that has deeply damaged both parties in similar situations in the past.

But given the opportunity and the magnitude of the problems facing the country, Democrats said they would welcome the chance  and the potential accountability  even though winning the White House is no assurance that their initiatives would sail through Congress. And all of the issues on their agenda may be overshadowed by the need for urgent action on the deepening financial crisis.

“I think we are in enough trouble in enough areas,” said Senator Sheldon Whitehouse, Democrat of Rhode Island, “that I would rather own it and then have to perform than continue with this back and forth, back and forth with Republicans, particularly while they are engaged in this absolute determined policy of obstruct, obstruct, obstruct.”