WASHINGTON - Democratic House leaders scrambled to round up enough support yesterday for their sweeping health care overhaul, getting last-minute help from White House advisers on the eve of a historic vote scheduled for today.

Amid signs of division in the Democratic ranks, majority leader Steny Hoyer acknowledged that Speaker Nancy Pelosi and her team had yet to secure the 218 commitments needed for passage. Obama, who picked up the phone and asked fence-sitting Democrats to get on board with his top domestic priority, delayed a trip to Capitol Hill until today. Representatives were put on notice that the floor vote may be postponed until tomorrow afternoon or even early next week.

"There are many people who are still looking to get a comfort level that this is the right thing to do,'' Hoyer said in a conference call with reporters yesterday.

There are 258 Democrats in the House, but a number of conservative "Blue Dog'' Democrats are against the bill or wavering because of concerns about raising taxes to pay for the insurance coverage expansion and about the scope of the plan. Some are also pausing over abortion and immigration, volatile side issues.

House leaders raced to satisfy those concerns, wary that they must strike a careful balance appeasing worried centrists without alienating pro-overhaul allies such as Hispanic lawmakers, who oppose the Blue Dogs' efforts to prevent illegal immigrants from buying insurance.

There were also signs of trouble from the left: Representative Eric Massa of New York, who supports a single-payer insurance system, announced that he planned to vote against the overhaul package, telling home state reporters that it would "enshrine in law the monopolistic powers of the private insurance industry.''

The White House said Obama put off his scheduled trip to the Capitol yesterday because of the Fort Hood shootings and because he wanted to go closer to the vote.

Even so, he and his top aides worked feverishly to help secure votes, arguing to moderate Democrats that they could improve the bill later but if it fails this weekend, the effort could collapse.

"There is definitely a sense of urgency that you can feel,'' said Representative Jason Altmire, a Blue Dog Democrat from western Pennsylvania who hadn't made up his mind. Altmire said he spoke with Obama for 10 minutes yesterday morning. He also took calls from Rahm Emanuel, the White House chief of staff, Health and Human Services Secretary Kathleen Sebelius, Education Secretary Arne Duncan, and a number of House leaders.