Representative Nancy Pelosi of California, the House minority leader, made several calls to key House Democrats to ensure the votes would be there in the House to sustain the president’s veto.

“The light of day has changed the way this agreement might look,” Mr. Brown said.

The standoff has set up a high-stakes game of chicken. Some 55 tax breaks for businesses and individuals expired last year, and if they are not revived retroactively by Dec. 31, taxpayers will not be able to claim them for the current tax year. Republicans say Congress might pass a one-year retroactive measure that would simply start the fight all over again in January, when they control the Senate and their numbers are fortified in the House.

The emerging tax legislation would make permanent 10 provisions, including an expanded research and development tax credit, which businesses and the Obama administration have wanted to make permanent for years; a measure allowing small businesses to deduct virtually any investment; the deduction for state and local sales taxes; the American Opportunity Tax Credit for college costs; deductions for employer-provided mass transit; and four different breaks for corporate and charitable giving.

Image Senator Sherrod Brown of Ohio said that the body’s Democrats would support the president if he made good on his veto threat. Credit... Daniel Rosenbaum for The New York Times

Smaller measures already passed by the Senate Finance Committee, from tax breaks for car-racing tracks to benefits for racehorse owners, would be extended for one year and retroactively renewed for the current tax year.

The tax credit for wind power, a Democratic priority, would phase out and end after 2017. Many conservatives oppose the wind credit as an unfair boost to an energy industry in competition with oil and gas.

Left off were the two tax breaks valued most by liberal Democrats: a permanently expanded earned-income credit and a child tax credit for the working poor. Friday night, Republican negotiators announced they would exclude those measures as payback for the president’s executive order on immigration, saying a surge of newly legalized workers would claim the credit, tax aides from both parties said.