President Reagan's chances of getting a tax-simplification bill through Congress look better than ever, leaders of both parties said yesterday, but the Republican Party's prospects of turning it into a breakthrough political issue have probably declined.

"The ball game is over," said former Senate majority leader Howard H. Baker Jr. (R-Tenn.), predicting congressional approval of a major tax bill "faster than most people expect. I have never seen such a confluence of important opinion on a major issue. When you get House Ways and Means Committee Chairman Dan Rostenkowski D-Ill. backstopping Ronald Reagan, you've got a combination that can't be beat."

Others on Capitol Hill were more cautious about the prospects, but the positive response reported by several congressional offices to both Reagan's and Rostenkowski's speeches Tuesday night clearly improved the legislative climate.

Critics of key provisions of the Reagan plan toned down their barbs yesterday, as the strength of the bipartisan drive for tax reform became clear. But both parties -- and their potential presidential contenders -- continued to maneuver for advantageous position.

Rallying behind Rostenkowski, whose televised response to Reagan's speech on tax simplification was praised by Baker and many others in both parties as a political master stroke, Democrats moved quickly to put their party stamp on the issue. Their special target was the middle-class constituency Reagan swiped from them in the 1980 and 1984 campaigns.

GOP strategists conceded that the Democrats had shown deft footwork -- thanks largely to Rostenkowski's speech -- but maintained that the GOP will be the ultimate beneficiary of the gathering political momentum behind the plan.

"Rostenkowski is a big winner," Baker said, "but there's going to be enough credit to go around. We Republicans will be able to brag about it, but so will the Democrats."

Reagan's initiative "reinforces the picture of the Republican Party as the party of action and the party of new ideas," said William Greener, political director of the Republican National Committee.

But a chorus of Democrats -- including almost all the party's prominent presidential possibilities -- picked up the Rostenkowski theme that Democrats had been there first on the issue of tax reform and that Democrats would be needed to hold the administration to its professed commitments.

Among those jumping aboard what looked like the tax-reform bandwagon were New York Gov. Mario M. Cuomo (D), a 1988 presidential possibility who is sharply critical of one key provision of the Reagan plan, and two House members with presidential ambitions and their own tax-simplification bills, Rep. Jack Kemp (R-N.Y.) and House Democratic Caucus Chairman Richard A. Gephardt (D-Mo.).

Cuomo, who has been the most vocal critic of the Reagan proposal to end deductibility of state and local taxes, nonetheless said yesterday that he is "very much for what the president is trying to accomplish in the overall bill . . . . It's what we have already done here in New York."

Kemp, who had said Tuesday that he "cannot support the plan as long as the top income tax rate remains at 35 percent," said yesterday that he was "not positioning myself as an opponent . . . . It's basically a good bill, and I want to make it better."

Kemp is feeling pressure from Cuomo to defend the tax-deductibility of state and local taxes -- the elimination of which would be more expensive to heavily taxed New Yorkers than to residents of almost any other state. At the same time, Kemp is a longtime tax-reform advocate who sees his potential 1988 rival, Vice President Bush, moving actively to identify himself with the potential success of the Reagan plan.

Bush is normally reticent about his role in administration policy debates, but his spokesman, Marlin Fitzwater, has told any reporter who asked that Bush argued actively for restoring some of the tax advantages to the oil and gas industry that had been ticketed for elimination in the original Treasury plan.

Gephardt, who had accused Reagan Tuesday of "sellouts" to special interests and of espousing "not tax reform, but tax retreat," reversed field yesterday and said the president "should be commended for a far-reaching and significant proposal."

That put him in line with Rostenkowski, with Sen. Bill Bradley (D-N.J.), cosponsor of the pioneering Bradley-Gephardt Democratic tax-simplification bill, and with such other Democratic headliners as Sen. Edward M. Kennedy (D-Mass.) and Sen. Gary Hart (D-Colo.).

Even while softening his initial sharp criticism of Reagan, Gephardt continued to insist that Republican concessions to business would deny middle-income taxpayers the full benefits of tax reform.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) struck that same theme in a statement saying that "these middle-income families pursuing the American dream -- the struggling class -- must be the No. 1 target of tax reform."

That line, launched by Rostenkowski in his televised reply to Reagan, represents a concerted Democratic effort to grab political dividends from the president's initiative.

To the extent that it succeeds, it could bolster what has been seen as a long-shot chance at best for Rostenkowski to challenge House Majority Leader James C. Wright Jr. (D-Tex.) for speaker when O'Neill steps down next year.