WASHINGTON - President Obama's hopes for a bipartisan health deal seemed in jeopardy yesterday as GOP senators protested his renewed support for a new public health insurance plan, and a key Democratic chairman declared that such a plan would likely be in the Senate's bill.

A public plan that would compete with private insurers is opposed by nearly all Republicans. Obama long has supported it, but he had avoided going into detail about his health goals, leaving the specifics to Congress and emphasizing hopes for a bipartisan bill.

That changed when Obama released a letter Wednesday to two Senate Democrats saying he believed strongly in the need for a new public plan.

"It wasn't helpful, it wasn't helpful," said Sen. Charles E. Grassley of Iowa, top Republican on the Senate Finance Committee, which is taking a lead role in crafting a health-care overhaul. "Words make a difference. And this made a difference."

"Didn't help. It hurt," said Sen. Mike Enzi of Wyoming, top Republican on the Senate Health, Education, Labor, and Pensions Committee.

The White House spokeswoman for health policy, Linda Douglass, responded: "As the president said in his letter, he remains hopeful that many Republicans will see fit to join with Democrats to enact legislation that will lower health-care costs for businesses, families, and government."

Congress might be able to pass a health overhaul with little, if any, GOP support. But Obama and Democrats including Finance Committee Chairman Max Baucus have said repeatedly they want to avoid that outcome because such a measure would be less widely supported and less sustainable over time.

Supporters of a new public plan contend it would give people more choices, create more competition, and "keep insurance companies honest," as Obama wrote to Baucus (D., Mont.) and Sen. Edward M. Kennedy (D., Mass.), chairman of the health committee.

Opponents say private insurers could not compete with a public plan that did not have to make a profit. They argue that private health plans would end up going out of business, leaving only an entirely government-run health-care system.

There appears to be little room for compromise, with Republicans contending that no matter how a public plan is designed, it would inevitably balloon and crush the private market.

"It's kind of a litmus-test sort of thing," Grassley said. "It's just very, very difficult, but I suppose that somewhere out there there's something that's politically realistic that's not a public option that satisfies Republicans and Democrats. But it isn't a government-run system."

Many Democrats, meanwhile, insist that a final bill must contain a public plan. Even Baucus, a moderate who has been working intensely with Grassley to produce a bipartisan product, said yesterday that he could not see the Senate passing a health-care overhaul without one.

"I think a bill that passes the Senate will have some version of a public option," Baucus said after the second of two meetings he convened yesterday on health legislation.

The president wants a bill on his desk in October.