President Carter probably will delay unveiling of his long-waited tax-revision package for at least another month - and possibly even longer, sources here said yesterday.

Although officials said the measure still could be announced in two weeks or so, sources described the President as reluctant to disclose his proposals until the energy bill is completed and Congress goes out of town.

At the same time, Carter reportedly has decided to toughen some of the tax-revision recommendations made by his Treasury advisers - particularly in the controversial area of eliminating special tax treatment of capital gains.

Sources say the toughening has been urged by Carter's close political advisers, including Stuart Eizenstat, his chief domestic aide, and Vice President Walter F. Mondale. Both are said to regard the Treasury proposals as too timid.

The developments came as Carter continued to work privately on the tax package, which he has had under consideration since Sept. 23. The proposal originally had been scheduled for unveiling Sept. 1.

The President met with his key economic advisers on Monday to review the Treasury recommendations but gave no indication then when he might make final decisions. The White House is said to be making sizable revisions.

The administration got a break on Tuesday when Rep. Al Ullman (D-Ore.), chairman of the House Ways and Means Committee, effectively released the Treasury from any obligation to send the proposal up before Congress adjourns.

Ullman talked to Carter by telephone o n Tuesday to make known his views on specific tax-revision proposals. The Ways and Means chairman also told the President then he would oppose any next tax cut for 1978.

The proposals the White House is said to be toughening involve a move to end special tax preferences for capital gains - the profits from the sale of securities or other assets - only half of which now are subject to tax.

Carter is reported to be insisting that the package include a proposal to tax capital gains accrued at the time a taxpayer dies, and to tighten some exceptions that the Treasury had recommended.

The President also is said to have overruled a Treasury request that he postpone his planned phaseout of two existing tax breaks for corporations doing business abroad.

W. Michael Blumenthal, the Secretary of the Treasury, had urged the President to go slowly in eliminating the provisions in view of the nation's mounting trade deficit problems. But Carter reportedly has demurred.

The provisions include the tax-subsidy for Domestic International Sales Corporations, which costs the Treasury $1.2 billion a year, and the ability of corporations to defer payment of taxes indefinitely on foreign-source income.

Carter listed both those provisions as targets for "tax reform" during the 1976 presidential campaign. The Treasury, and most private tax experts as well, argued the DISC subsidy was not effective in promoting exports.

It was not immediately clear how further delay of the tax-revision package would affect its prospects for passage. Although some experts say it won't make any difference, others contend the move could hurt the package.