WASHINGTON - The Obama administration's bottom line on a government health insurance option blurred Sunday as White House officials stressed support but stopped of short of calling it a must-have part of an overhaul.

As President Barack Obama prepares for a Wednesday night speech to Congress in a risky bid to salvage his top domestic priority, no other issue is so highly charged. Obama's liberal supporters consider the proposal for a public plan to compete with private insurers do-or-die. Republicans say it's unacceptable. It's doubtful the public plan can pass the Senate.

White House political adviser David Axelrod said Obama is "not walking away" from a public plan. But asked if the president would veto a bill that came to him without the option, Axelrod declined to answer.

The president "believes it should be in the plan, and he expects to be in the plan, and that's our position," Axelrod told The Associated Press.

Asked if that means a public plan has to be in the bill for Obama to sign it, Axelrod responded: "I'm not going to deal in hypotheticals. ... He believes it's important."

The biggest challenge Obama faces in his prime-time address is to take ownership of health care legislation that until now has been shaped by political conflicts in Congress. Lawmakers return this week from a summer break that saw eroding public support for an overhaul and contentious town hall meetings in their districts.

The idea of a public plan has become a symbol for the reach of government in a revamped health care system. Supporters say it would give workers and their families similarly secure benefits as older people now get through Medicare, while leaving medical decisions up to doctors and patients. The plan would be offered alongside private coverage through a new kind of purchasing pool called an insurance exchange. At least initially, the exchange would be open to small employers and people buying coverage on their own.

Insurers say they could never compete against the price-setting power of government. Employer groups warn it would undermine the system of job-based coverage.

A public option -- or government plan -- has come to mean different things to different people. Some say it could be a public trust and independent of the government: non-profit co-ops could serve as a check on insurance companies. In its original form, supporters envisioned a Medicare-like plan in which the government pays the bills. But it would be financed through premiums paid by beneficiaries, not taxpayer dollars.

While there's strong support for a public plan among House Democrats, the votes appear to be lacking in the Senate.

Democratic Sen. Ben Nelson of Nebraska, a swing vote on health care legislation, said the only way a public plan should be included is as a last resort. The government option would only be rolled out if after a few years, private insurers have failed to increase competition and restrain costs.

"If somehow the private market doesn't respond the way it's supposed to, then it would trigger a public option, or a government-run option," Nelson said on "State of the Union" on CNN. "But only as a fail-safe backstop."

White House spokesman Robert Gibbs, who joined Axelrod in a one-two administration punch on the Sunday talk shows, said the president believes a government plan would be "a valuable tool." But Gibbs danced around the question of whether it has to be in the final legislation.

"We're not going to prejudge what the process will be when we sign a bill, which the president expects to do this year," he said on ABC's "This Week."

The uncertainty over the administration's position isn't new. Health and Human Services Secretary Kathleen Sebelius said last month that a government alternative to private insurance is "not the essential element" in revamping the system to guarantee coverage for all and try to curb unsustainable costs.

On Friday, during a call with prominent liberal House members, Obama refused to be pinned down on the public plan, a participant told The Associated Press. "It was unclear as to whether the public option is on or off the table," said the official, who spoke on condition of anonymity because of the sensitivity of the issue.

Independents who helped propel Obama to the presidency are increasingly skeptical about his direction on health care. Unsubstantiated allegations that the legislation would promote euthanasia grabbed headlines last month. But beneath such controversies, voters appear most concerned about the scope and costs of the bill -- around $1 trillion over 10 years. Obama has said he won't sign a bill unless it is fully paid for and doesn't add to the deficit.