The Democratic Party executive committee today condemned President Reagan's tax bill as "a welfare program for the wealthy" and called on congressional Democats to hold the line for their Party's alternative.

Meeting as Reagan was wooing southern Democratic support for his three-year, 25 percent across-the-board tax cut, the party's executive committee said the president's proposal would fuel inflation, increase the federal deficit and shift income from "moderate and middle-income Americans to the privileged few."

But the debate that preceded the unanimous approval also displayed the party leader's frustration with those House Democrats whose votes gave Reagan his victory on the budget and could do so again on the tax bill.

Noting that "the president cannot push his program through Congress without the cooperation and collaboration of members of the Democratic party," Gary Mayor Richard Hatcher said, "It is time to say . . . that we are watching very carefully and we will not forget those who choose to abandon the principles of the Democratic Party."

The resolution endorsed the alternative two-year, 15 percent tax cut package proposed by House Ways amd Means Committee Chairman Dan Rostenkowsi (D-Ill.) as one "that truly serves the needs of the average working family and increases noninflationary investment and job opportunities."

The executive committee also condemned Reagan's proposed reductions in Social Security benefits and the administration's decision to sell sophisticated weapons to Saudi Arabia.

All three resolutions will go to the full Democratic National Committee tomorrow for expected ratification.

While the executive committee was declaring itself on those policy questions, it also cleared away the last procedural preliminaries to a decision to shrink the size and tighten the membership qualifications for the 1982 midterm party conference.

With minimal debate, the executive committee approved a revision that will cut the conference in half and have its delegates chosen by state and national committees, not elected at the grass-roots.

Critics of the change at today's meeting, including representatives of the National Educational Association and party reformers from Wisconsin, New York and California, argued that it was a step away from the participatory politics mandated by party rules in effect since 1968.

Because of the tensions, party chairman Charles Manatt decided to delay naming the members of the commission that will re-examine 1984 delegate-selection and nominating convention procedures, under the chairmanship of North Carolina Gov. James Hunt.Aides said more than 100 people have asked to be on the commission. It will probably be kept to half that size, but balancing its makeup poses a challenge to the new party chairman.

Manatt will announce tomorrow that he has picked former House member Yvonne Brathwaite Burke of California to head a commission on platform accountability, which was mandated at the last convention. That is also a politically sensitive post, since the commission is supposed to report to the party on how well elected Democrats worked to fulfill the platform's pledges.

Manatt has rejected any suggestion that the party will attempt to censure or discipline House members who supported the Reagan Budget, for example, despite the complaints of some party constitutencies about them. In giving the chairmanship to Burke, Manatt made a gesture to blacks and women and still picked someone with close links to congressional Democrats. Serving as co-chairmen with Burke will be Terry Herndon, executive director of the National Education Association, and Gov. William Waller of Mississippi