House Republicans, working closely with both President Reagan and conservative Democrats, yesterday sharply scaled back their planned alternative to Democratic-drafted spending cuts in hopes of putting together a winnable package by today.

Their final drive for a compromise came as House Speaker Thomas P. (Tip) O'Neil Jr. (D-Mass.) asserted that they lack the votes to duplicate their stunning victory last month in getting the Democratic-controlled House to go along with Reagan's budget targets, almost to the last detail.

O'Neil contended that Minority Leader Robert H. Michel (R-Ill.) was being "forced" into backing a substitute for the cuts voted by the Democratic-dominated House committees. "I feel sorry for Bob," said O'Neil. "As I see it, he's just being given the paper and told, 'Here, pass this.'"

Michel laughed off O'Neil's commiserations, but acknowledged he faces a tougher job this time and is limiting the counter-proposal largely to changes in open-ended entitlement programs, rather that cuts in programs that are authorized for specific amounts of money.

Some Republicans are decidedly unenthusiastic about joining the White House in a possibly risky drive to bring spending cuts proposed by House committees more in line with Reagan's specific budget-cutting proposals, including his plan to consolidate many programs into block grants to the states. Moreover, even Michel concedes that many of the conservative Democrats who helped pass Reagan's budget last month are reluctant to vote against cuts that were approved by House committees.

Michel declined to discuss details of his planned package, saying they were "still a matter of delicate negotation," but insisted that Reagan and his House supporters were highly likely to have a plan in hand by today. The final word on whether to push ahead rests with Reagan, Michel said after a day-long series of meetings that included a working lunch with the president at the White House and subsequent strategy sessions with House Democrats and Republicans, including Reps. Charles W. Stenholm (D-Tex.) and G.v. (Sonny) Montgomery (D-Miss.), leaders of the conservative Democratic caucus in the House.

Meanwhile, the House Budget Committee added one more "sweetener" to the panel's proposals in an acrimonious session during which Republicans protested in vain the Democrats' refusal to allow a vote in the normal way on the GOP substitute.

The budget unit deleted a highly controversial proposal calling on the Postal Service to draw up plans for shutting up to 10,000 small post offices. Although the Budget Committee is barred from altering the committee's proposals, it fashioned a proposed rule for floor action in such a way that a crucial procedural vote would save the post offices.

"We want to take out any sham targets from the substantive debate," said Budget Committee Chairman James R. Jones (D-Okla.)

In action that was subsequently ridiculed by the administration, the House Post Office and Civil Service Committee had proposed saving $100 million by directing the postmaster general to prepare a shutdown plan, subject to congressional veto. The Budget Committee retained the $100 million savings but left it to the Postal Service to decide where to cut.

The post office shutdown was among a number of provocative cuts proposed by committees to mobilize support for amendments on the House floor to restore money, but it backfired when the Republicians weighed in with their substitute. To cut off the Republicans, the Democrats had to shut off all amendments, forcing the committees to reshuffle the cuts into a more acceptable form.

A recommendation for a "closed rule," banning all but procedurally necessary amendments, was a foregone conclusion from the Budget Committee. But both sides nonetheless staged a sometime angry debate over whether the Republicans should be allowed to offer their substitute without resorting to parliamentary backdoors.

Democrats protested that the Republicans were asking for a "blank check" for legislation that was still a mystery and that the White House was trying to "dictate" the details of legislation to Congress even after its committees had met Reagan's general budget-cutting target.

The Republicans and their conservative Democratic ally, Rep. Phil Gramm (D-Tex.) countered that the Democrats were trying to block the House from making a legitimate legislative choice.