Appropriators from both parties and both sides of the Capitol have opened intentionally secretive negotiations on the mammoth and complex measure necessary to make good on the budgetary truce just called by Congress.

The four-dozen or so members involved have given themselves less than three weeks to agree on the several thousand line items in the bill, which will be written as non-amendable legislation dictating all of the government’s discretionary spending for the final 37 weeks of this budget year.

The enormity of the task and the extraordinarily tight time table would normally present significant obstacles to a smooth or successful outcome. But the lawmakers who have taken the assignment are betting that those challenges will be eased by several factors:

• The fiscal deal the Senate cleared Thursday, which President Barack Obama will sign before leaving this weekend to spend the holidays in Hawaii, sets a grand total of $1.012 trillion for the package that both parties’ negotiators say they can live with. The figure is $45 billion, or 4.6 percent, more than would have been allowed if the sequester had remained fully on the books.
• The vast majority of lawmakers, not to mention hundreds of lobbyists and advocates, will be away from Washington during the next two work weeks. That should afford the negotiators and their aides an opportunity to set their priorities and make their tradeoffs without the usual volume of importuning — a tiny silver lining, also, for having to work through Christmas and New Year’s Eve.
• The leaders of the talks, Kentucky’s Harold Rogers for the Republican majority in the House and Maryland’s Barbara A. Mikulski for the Democratic majority in the Senate, have agreed to draft the bill as a take-it-or-leave it package deal. Their bet here is that substantial numbers from the rank and file in all four caucuses will be willing to set aside their reservations about the content and their annoyance about the process and vote “yes” — because they know defeating the measure would threaten another government shutdown as the first congressional action of the midterm election year.

The timetable for the next three weeks sketched by Rogers and Mikulski, the chairmen of the two Appropriations committees, begins with a deadline they have set for themselves for the end of the week: apportioning the spending grand total into a dozen slices — the so-called 302(b) spending caps for the subcommittees that are supposed to write 12 different spending bills every year.

This is an immensely important first step, because it means choosing winners and losers at a macro level. A relatively generous number for the subcommittees with jurisdiction over labor, health and education programs, for example, would provide some relief from social spending limits that Democrats would embrace. But that money would mean somewhat smaller top lines for subcommittees in charge of the domestic programs Republicans typically favor, which cover such things as water projects, law enforcement and homeland security.

There was word Thursday evening that they had come to an agreement. But, as an indication of the sensitivity of these initial decisions, Rogers and Mikulski have decided to break with longstanding practice by not making public the 302(b) allocations.

Instead, the top Democrats and Republicans on each House and Senate subcommittee will be told of their cap and then given until Jan. 2 to come up with as much of a plan as they can agree on for dividing that pot of money and altering any policies along the way.

The challenge will impose especially dicey political challenges on three senior Republicans: Thad Cochran of Mississippi, his party’s second-most-senior Senate appropriator, faces an intense primary challenge on his right next year, as does Mike Simpson of Idaho, a subcommittee chairman in the House. Jack Kingston, the third-most-senior House appropriator, is in a hot primary against several fiscal conservatives for Georgia’s open Senate seat. All will face pressure to use their work on the bill as a way to say they’re cutting excessive spending and otherwise tacking to the right.

As a starting point, the appropriators will all presumably use the bills they advanced to various stages of completion earlier this year. Eleven bills got through Senate Appropriations, but none was passed on the floor. The House passed four of its bills, but only five others won endorsement from the full Appropriations Committee.

Because of the way the sequester law works, measures related to national defense have their own limit, now $520 billion for this year — more than before this month’s deal to ease the across-the-board limits, but still about $30 billion less than what House and Senate appropriators wrote into their bills several months ago. The limit on all non-defense spending for fiscal 2014 is $492 billion — the “pie” over which the bulk of the haggling will take place.

In addition to spending totals, any compromise omnibus would have to resolve intense and partisan differences on the use of federal funds to implement an array of policies — starting with, but hardly limited to, the health care law, environmental regulations and the rules to carry out the Wall Street oversight law. Some appropriators may want to push for legislative riders to address matters that have cropped up since the regular spending process stalled out this summer — for example, by making sure all the revenue from the budget deal’s new airline ticket fee goes to aviation security.

Any programmatic totals or policy disagreements that still remain after the first weekend in the new year will be taken out of the hands of the subcommittees and turned over to the big four appropriators: Mikulski, Rogers, top Senate Appropriations Republican Richard C. Shelby of Alabama and top House Appropriations Democrat Nita M. Lowey of New York.

The quartet will convene early the week of Jan. 6, when Congress reconvenes for the new year, with a goal of finalizing the entire package quickly enough that it can be put before the House for an up-or-down vote by Friday, Jan. 10. (The parliamentary sleight-of-hand that will be deployed to move the package along without amendment has not yet been settled on.)

That would allow the Senate to begin debating the legislation Jan. 13 and – assuming no one insists on a filibuster-busting cloture vote — send it to Obama before the current stopgap continuing resolution lapses at midnight on Jan. 15.

For now, the lawmakers driving the process are refusing to countenance the notion that the schedule is too unforgiving, and that another CR might be needed to patch the budget for a while beyond the middle of January.

“We’re all up to the task,” Mikulski told reporters Wednesday. “Our problem is it’s a very tight timeline.”