Senior Reagan administration officials began selling a two-year, $76 billion deficit-reduction accord to Senate Republicans yesterday as Congress prepared for an arduous struggle to write the specifics of the agreement into law.

During a lunchtime meeting with Republican senators, White House chief of staff Howard H. Baker Jr. and Treasury Secretary James A. Baker III appeared to disarm some opponents of the budget pact reached Nov. 20 by President Reagan and congressional leaders.

"The budget summit package is likely to be adopted when all is said and done," predicted Sen. Bob Packwood (Ore.), ranking Republican on the Senate Finance Committee, who had earlier sharply criticized the plan as too modest.

Packwood and other senators who have faulted the agreement as a flawed compromise unlikely to bolster confidence on world financial markets conceded yesterday that a combination of procedural hurdles and time constraints would stymie any attempts to craft a more ambitious packge.

"The leadership is putting on a very strong push for its package," said Sen. Kent Conrad (D-N.D.). "That will make any amendments to expand it very difficult."

Sen. Joseph R. Biden Jr. (D-Del.), a leader of a bipartisan group that has proposed an alternative plan to freeze federal spending at last year's levels, said yesterday he would not push that proposal unless the leadership package founders.

With administration officials pushing the plan as one the president strongly supports, Sen. Pete V. Domenici (R-N.M.) said he is "more and more confident" that a "compelling majority of senators" will vote for it.

The grudging acceptance of the pact developed as the House and Senate prepared the groundwork for two bills needed to translate the spending cuts and tax increases called for in the agreement into law.

On the Senate side, Finance Committee members began reviewing options for raising the $9 billion in new revenues needed to achieve the first-year goal of reducing the deficit by $30.2 billion. Chairman Lloyd Bentsen (D-Tex.) said that by Friday the committee will produce a bill including the taxes and some permanent reductions in spending for programs such as Medicare.

Bentsen said the bulk of the $9 billion would likely come from provisions common to the committee's earlier tax bill and one passed by the House. Both those bills, which each totaled about $12 billion, were largely targeted at corporations and wealthy individuals.

But administration opposition to several key components of the Senate bill, including a $2.2 billion provision lifting the cap on Medicare payroll taxes, could force the committee to go beyond the scope of its original bill.

In the House, meanwhile, Democrats and Republicans prepared for what may be a bitter fight over the second measure needed to implement the pact, an omnibus spending bill that will eventually incorporate almost $8 billion in discretionary spending cuts involving defense and domestic programs.

The legislation, known as a continuing resolution, is scheduled for a House vote Thursday. Prospects for passage may be clouded by disputes over extraneous issues. Democratic amendments would write into law the "fairness doctrine" requiring broadcasters to air all sides of controversial issues and would postpone a clean air deadline.