The House Ways and Means Committee is scheduled to begin formal markup of President Carter's tax package today. Prospects are that the panel will major changes in the tax-reduction and "reform" proposals the president sent up in January.

After a month of tedious hearings, the committee has laid out a time-table aimed a sending a bill to the House floor by early June. The major question: how much of the Carter package will survive intact? Also: can the panel meet its deadline.

Carter proposed a massive $24.5 billion tax reduction and "reform" program, comprising $33.9 billion in tax cuts for business and industry offset by $9.4 billion in revenue-raising tax "reforms." Reaction in January was mixed, but the tax cuts were hailed as needed.

Since then, however, the committee has been buffeted by conflicting political pressures that are seriously threatening both the schope and content of the entire Carter tax package - the proposals for tax reductions as well as the "reforms."

Althought the outcome is uncertain, many key members now expect the panel to cut the tax-reduction package substantially, and to skew proportionally more of the tax relief to middle and upper-middle-income taxpayers.

In addition, congressional observers say the committee may substitute a rollback in Social Security taxes for some of the income tax cuts the president has proposed. Most of the proposals for "tax reform" are likely to be scrapped.

Among the new forces that have influenced the committee over the past few weeks:

Concern among House members about the need to hold down the federal budget deficit in the inflation fight.

As key committee members think about paring the tax cut from the $24.5 billion the president proposed, Democrat Charles A. Vanik of Ohio is preparing a proposal that would scrap that tax cut entirely and simply extend the modest tax reductions that were enacted in 1977. While that may not pass, insiders say the vote may be close.

Continuing demands by a large proportion of House members for a rollback or reduction in Social Security payroll taxes - an issue regarded by some congressmen as a key element in their chances for re-election.

The caucus of all House Democrats and the House Budget Committee are on record as strongly favoring some sort of payroll tax relief. And whatever the committee does is almost certain to come at the expense of the income tax cuts Carter proposed.

A desire by many members to channel a larger share of whatever tax relief is adopted to middle and upper-middle incomes - taxpayers in the $17,000 to $35,000 brackets, who are mot articulate and politically vocal.

One likely result, most observers say, is that the panel will scrap Carter's proposal to replace the familiar $750 a-dependent personal exemption with a $240-a-person tax credit. While the credit was designed to benefit the poor, some members fear it would penalize the middle class.

Continuing lack of enthisiasm among committee members of virtually any of Carter's $9 billion in proposed "reforms," including the big three that Carter has been emphasizing - ending two foreign tax breaks and limiting deductions for the "three-martin" business lunch.

One added problem: If the panel rejects as much of the "reform" package as members now are predicting, it will have to reduce the tax-reduction part even further, to prevent bloating the budget deficit.

The situation is complicated even further by the fact that Democrats on the Ways and Means Committee are so badly split themselves that neither Carter nor the congressional leadership can count on the traditional majority in support of the bill's key provisions.

Indeed, it is the liberals on the panel who now seem most opposed to the president's "tax reform" proposals and most concerned about the budget deficit. The Republican minority is expected to push for larger tax cuts - in part to embarrass Carter.

Rep. Al Ullman (d-Ore.), chairman of the committee, has tried to uphold the administration's position in the early stages of the panel's hearings, but has been overriden, first by the Democratic caucus (on the Social Security issue) and then by the panel's Democrats.

Even the administration has shown signs of split on the tax issue. Carter on Friday scotched the idea of reducing the size of the tax cut, which some key advisers - notably Secretary of the Treasury W. Michael Blumenthal - had been advocating.

While Blumenthal since has jumped back on the bandwagon, the schism hurt the administration's position. To boot, G. William Miller, the man Carter appointed as chairman of the Federal Reserve Board, also has been urging that Congress reduce the size of the tax cut.

As it stands now, the outlook for the tax package at best is uncertain. The administration has been trying desperately to keep the program together, contending that to dismember it would cost "up to 1 million additonal jobs by 1979."

But the tax package is so fragile in its structure - so many key provisions dependent on one another - that observers now are skeptical it will survive intact.

The schedule calls for the panel to take up the "reform" proposals first and then on to the President's tax-reduction measures - presumably after scrapping most of the "reforms." In between, it no doubt will stage a fight over Social Security.