Millions of Obamacare enrollees would lose health coverage under the Republicans’ major policy bill, which would make coverage more expensive and harder to obtain.

Most of the proposals in the bill, which passed the House last month, are technical changes — reductions to enrollment periods, adjustments to formulas, and additional paperwork requirements. But together, they would leave about four million people uninsured in the next 10 years, the Congressional Budget Office reported Wednesday.

“In many ways, it’s sort of repeal by paper cut,” said Audrey Morse Gasteier, the director of the state marketplace in Massachusetts.

Alongside these proposals is another challenge to the program: Additional Obamacare funding is set to expire at the end of the year, and Republicans do not plan to extend it. If they don’t, the C.B.O. estimates an additional 4.2 million Americans would lose coverage.

Taken together, proposed changes and the expiration of the subsidies could threaten the viability of the Obamacare markets themselves, which have more than tripled in size since 2014, and currently cover 24 million people.