Among its most significant parts, the proposed deal on the so-called tax extenders would make permanent a tax credit for corporations’ research and development expenses that dates to 1981. That credit has been extended more than a dozen times, but for no more than a year or two — largely to minimize the official price tag of its extension. A popular write-off for small businesses’ major purchases also would be made permanent.

 

President Obama and Democrats in Congress have demanded in return that the tax package make permanent three provisions that were among the president’s first-term economic stimulus measures: an expanded earned-income tax credit for low-wage workers, an expanded child tax credit and a credit for four-year college costs.

 

“I sense a real appetite for a balanced deal that incorporates priorities the Democrats have, priorities that Republicans have, that will provide a measure of certainty and predictability to the economy, and provide growth and the ability for working families to get ahead,” Senator Ron Wyden of Oregon, the senior Democrat on the tax-writing Senate Finance Committee, said in an interview on Friday.

 

“All sides agree that this has to be a multiyear bill,” Mr. Wyden added, to give more predictability to businesses and individual taxpayers.

 

But a multiyear measure making a number of tax breaks permanent would also raise the cost of the legislation. The research and development credit, the single costliest business item, would reduce revenues by about $110 billion over the first decade.

 

The centrist group Committee for a Responsible Federal Budget has put the total package’s expense at about $700 billion — a sum that people familiar with the negotiations did not dispute — or about $840 billion counting interest on the additional federal debt. The group has called the emerging package “a fiscally irresponsible bipartisan deal.”

 

For decades, Republicans as well as many Democrats, including Mr. Obama, have called for making some of the tax extenders permanent, including the research credit. But they could not agree on offsetting savings, mainly because Republicans refused to increase other taxes

 

Now, with both parties colluding to waive the pay-as-you-go principle that legislation should not increase annual deficits, the negotiators from the House and Senate tax committees — in consultation with congressional leaders and the Obama administration — have bid up the size of the package over days of talks.

 

 

Negotiators also are considering whether to suspend for two years another tax in the health insurance law, a levy on medical device manufacturers.

 

By week’s end, people in both parties suggested the tax package was becoming a Christmas tree that could fall of its own weight. Supporters argue that many of the tax breaks have been on the books for years, but that short-term extensions have masked their long-term cost. Making them permanent and conceding the revenue loss upfront would amount to more honest budgeting, supporters say.

 

To critics among budget watchdogs and some Democrats, the deal, if it comes together, could set a troubling double standard, with only new spending — but not tax breaks — requiring offsetting budget cuts to avoid adding to deficits. Republicans, who now control both houses of Congress, have long argued that tax cuts do not need to be paid for with budget cuts. Now Democrats are going along in return for Republicans’ dropping their opposition to tax breaks for the working poor.

 

In contrast, Congress and the White House agreed to a bipartisan deal for new spending in October, and a highway measure this week, only after hard-fought compromises on budget cuts to pay for the spending increases.

 

Some Democrats in the House and Senate have registered protests, although a meeting of Senate Democrats on Thursday showed greater support for the emerging package than many expected. Among the opponents is Senator Mark Warner, Democrat of Virginia and a member of the Finance Committee, who recalled the hard fights to win a bit more spending for education, infrastructure, medical research.

 

“Yet people now are talking about spending $800 billion in lost revenue — that’s spending,” Mr. Warner said. “For those of us who think there are key areas where government needs to invest more — you know, education, infrastructure, research — if we spend the money on tax relief, on a so-called tax extender package, then it’s hard to make the case that we can spend money on other key priorities.”

 

Senator Tammy Baldwin, Democrat of Wisconsin, sent a letter to Mr. Wyden signed by her and four other senators calling for the tax bill to be paid for by curbing tax breaks for hedge-fund managers and corporate executives’ compensation.

 

The Republican Study Committee in the House, which includes most Republicans there, circulated its demands to end tax breaks for renewable energy, including wind and solar projects, and to require proof of legal status before workers can claim tax breaks. Democrats are protecting the renewable energy provisions, but discussing ways to address Republicans’ concerns about undocumented workers getting tax breaks.

 

The tax package would phase out over five years a bonus depreciation tax break for corporations, which was intended as a short-term stimulus measure, and two provisions that benefit foreign corporations and American multinational corporations. Republicans wanted to make the corporate breaks permanent, but Democrats and the administration objected, especially given the recent controversy involving Pfizer and other corporations that have acquired foreign companies to relocate in countries with lower corporate taxes.

 

A tentative trade-off would have Republicans agreeing to the five-year phaseout of the corporate tax breaks they favor in return for Democrats dropping their insistence on indexing the child tax credit so it increases in value with inflation. But House Democrats, led by Representative Nancy Pelosi of California, have been insisting on indexing the child credit.