Washington ? The chairman of the Senate Finance Committee said Monday that he will propose an overhaul of the nation’s health care system that addresses a host of GOP concerns, including blocking illegal immigrants from gaining access to subsidized insurance, urging limits on medical malpractice lawsuits and banning federal subsidies for abortion.

But even after Max Baucus, D-Mont., spoke optimistically of gaining bipartisan backing, lawmakers continued to haggle over a question at the heart of the debate: How can the government force people to buy insurance without imposing a huge new financial burden on millions of middle-class Americans?

Even within his own party, Baucus confronted a fresh wave of concern about affordability. Sen. Ron Wyden, D-Ore., declared himself dissatisfied with the chairman’s plan, which, like other congressional reform proposals, would require every American to buy health insurance by 2013.

“Additional steps are going to have to be taken to make coverage more affordable,” Wyden said, “and my sense is that will be a concern to members on both sides of the aisle.”

Under the Baucus plan, described in a “framework” he released last week, as many as 4 million of the 46 million people who are currently uninsured would be required to buy coverage on their own, without government help, by some estimates. Millions more would qualify for federal tax credits, but could still end up paying as much as 13 percent of their income for insurance premiums — far more than most Americans now pay for coverage.

People further down the income scale would receive much bigger tax credits, effectively limiting their premiums at 3 percent of their earnings. But experts on affordability say even those families could find it difficult to meet the new mandate without straining their wallets.

“We’re talking about the equivalent of a middle-class tax increase,” said Michael D. Tanner, a health care expert at the libertarian Cato Institute. “Yes, they’re paying it to an insurance company instead of to the government. But, suddenly, these people are paying more money to somebody.”

A plan drafted by House Democratic leaders would offer more generous tax credits, but it would cost more than $1 trillion over the next decade.

Baucus’s team of three Democrats and three Republicans from the Finance Committee has labored for months to cut that cost as it crafts a reform plan that could win support from both parties. By squeezing the size and scope of the subsidies, the negotiators have lowered the cost to a more politically palatable $880 billion — within the range President Barack Obama specified last week in a speech to Congress.

But a smaller bill would mean less help for people — particularly those who earn too much to qualify for Medicaid but too little to easily slip the equivalent of a second rent payment into their budgets.

According to the latest census data, about three-quarters of the uninsured earn less than 300 percent of the poverty level, or about $32,500 for an individual and $66,150 for a family of four. Nearly half are childless adults. In surveys, many say that they are not offered coverage by their employers or that they simply cannot afford it.