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.

Among other provisions under consideration is one opposed by the White House — yet favored by many Democrats and unions — that would delay for two years, until 2020, a tax on generous “Cadillac” health insurance plans provided by employers. The tax was included in Mr. Obama’s Affordable Care Act to help pay for the insurance program and to reduce health care spending over all by encouraging consumers to be more cost-conscious.

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.