WASHINGTON — Senators struggled Wednesday with the possibility that in offering subsidized health insurance to millions of individuals and families, they could inadvertently speed the erosion of employer-provided coverage, which they want to preserve.

Senator Max Baucus, Democrat of Montana, who is leading efforts to write health legislation, said “much of the discussion” focused on this issue at meetings of senators on Wednesday.

And, in the evening, in an indication of how the administration is going all out to sell the need for a new health care system, President Obama participated in a town meeting-style gathering televised by ABC News from the White House.

As part of an ambitious plan to overhaul the health care system, Democrats in both houses of Congress want to require people to carry insurance. They would offer subsidies to low- and moderate-income people who buy coverage through a new regulated market known as a health insurance exchange. Employers who do not provide coverage might have to pay penalties or contribute to a government fund.

Higher penalties are more effective in preventing the erosion of employer-sponsored coverage, the bedrock of insurance for more than 150 million Americans, the Congressional Budget Office told lawmakers.

Small businesses are less likely to provide health insurance than larger businesses. So any penalty for not providing insurance could affect them more than other employers.

Senator Blanche Lincoln, Democrat of Arkansas, said preserving employer-sponsored insurance “needs to be a huge objective.”

Senator Kent Conrad, Democrat of North Dakota, said Congress faced “a very complicated calculus” in devising the new requirements.

“Even with an individual mandate,” Mr. Conrad said, “if there is not some requirement for employers, you will see a drifting of people to the insurance exchange and to government subsidies. That will increase the cost.”

Mr. Baucus, the chairman of the Finance Committee, said senators were investigating “the ramifications and implications” of requiring employers to provide or pay for coverage, the so-called play-or-pay requirement. Mr. Baucus said he and other senators wanted to minimize the chance that employees “may be enticed to leave their firms in order to get health insurance in the exchange.”

The budget office said: “The availability of subsidized coverage in the new insurance exchange would be an attractive option for many lower-income workers. As a result, some employers would decide not to offer their employees health insurance coverage, opting instead to provide other forms of compensation.”

In a letter to Congress three weeks ago, President Obama said small businesses “should be exempted” from any employer mandate. Whether Congress will go along is unclear.

“We will give assistance to small business through tax credits,” Mr. Baucus said Wednesday.

When asked about an exemption for small businesses, Mr. Baucus said: “We talked about it. But how much sense does that really make?”

A bill drafted by House Democratic leaders says, “There will be an exemption for certain small businesses,” but gives no details.

Businesses generally oppose the play-or-pay requirement. They object, in particular, to the House Democrats’ bill, under which most employers would have to provide coverage or pay a fee equivalent to 8 percent of their payroll.

The United States Chamber of Commerce and the National Federation of Independent Business, which represents small employers, said the proposed requirement amounted to a new tax and would frustrate the creation of jobs.

At the televised session on Wednesday evening, Mr. Obama took questions from a panel of doctors and patients, saying the varying plans in Congress were coming together.

“We have to have the courage and the willingness to cooperate and compromise in order to make this happen,” Mr. Obama said. “And if we do, it’s not going to be a completely smooth ride. There’s going to be times over the next several months where we think health care is dead, it’s not going to happen. But if we keep our eye on the prize, then I’m absolutely convinced that we can get it done this time.”

Mr. Obama also left the door open to a new tax on health care benefits. He said he did not want to “prejudge” what efforts were being made to reach a compromise, including proposals in the Senate to tax workers who get expensive insurance policies. He opposed the tax as a presidential candidate.