To express his displeasure, Mr. Harkin has blocked Senate action on Mr. Obama’s nominee to be administrator of the Centers for Medicare and Medicaid Services, Marilyn B. Tavenner. By putting a “hold” on the nomination, aides said, Mr. Harkin hopes to draw the White House into negotiations on the future of the prevention fund, which he has championed.

At Congressional hearings this week, Kathleen Sebelius, the secretary of health and human services, said it was necessary to tap the prevention fund because Congress had refused to provide money requested by the president for outreach and education activities.

Senator Max Baucus, Democrat of Montana and chairman of the Finance Committee, said last week that the administration deserved “a failing grade” for its efforts to explain the law to the public.

“I just see a huge train wreck coming down,” Mr. Baucus said then.

But after hearing White House officials on Thursday, Mr. Baucus said he was encouraged, and he praised the administration’s efforts to get healthy young people to sign up for insurance coverage.

Senator Benjamin L. Cardin, Democrat of Maryland, said he told White House officials on Thursday that he was concerned about big rate increases being sought by the largest health insurer in his state. The company, CareFirst BlueCross BlueShield, has sought increases averaging 25 percent for individual insurance policies that will be sold in the state insurance exchange, and it is seeking increases of about 15 percent for small businesses. The company said the higher premiums reflected costs of complying with the new law.

Senator Cardin said he was also distressed by the administration’s failure to require health insurers to provide affordable coverage of dental services for children. The law lists pediatric dental care as one of 10 categories of “essential health benefits” to be provided by all health plans.