The Carter administrations, facing congressional skepticism over its coming tax revision program, is considering paring back its tax package to make it more easily "digestible" - and politically marketable - to Congress.

Although officials caution that the cutback is far from decided, the White House nevertheless is said to be seriously mulling the idea, which was suggested by congressional leaders.

The plan would split the President's tax revision package in two - with one part earmarked for consideration by Congress in 1978, and a second portion to be taken up in 1979 and later.

The initial portion would include some of the basic "reforms" Carter promised in the 1976 presidential campaign, and most - likely - an immediate tax cut to bolster the economy.

The second part would seek longer-range tax reductions and propose a spate of other, more minor revisions in the tax code that Carter strategists would like to see passed.

Administration sources stressed that the move to split the package still is only an outside possibility. One source indicated Carter was reluctant to take such a step unless he felt it was necessary to avert chaos later.

The split-up has been recommended by Democratic congressional leaders, some of whom are fearful the comprehensive package officials have outlined so far may be too much for Congress to handle in a single session.

Rep. Al Ullman (D-Ore.), chairman of the House Ways and Means Committee, has suggested his panel "may have to bobtail" the President's tax package in order to push it through next year, especially if Carter seeks new cuts.

However, some administration officials are known to fear that if the tax package is split, it could dim prospects that the second portion would be passed.

At the same time, "tax reform" advocates have been urging the President to propose the most comphrehensive package he can, in order to provide "leadership" to help achieve passage of the most-sweeping program possible.

Carter is not expected to make a final decision on strategy until Congress nears completion of its work on the energy bill, which is still uncertain. Current plans are to unveil the tax bill before Congress goes home this session.

There was no immediate indication of which of the Carter tax revision proposals would be included in the initial segment if the administration decided to split the package.

However, sources speculated the portion slated for passage in 1978 almost certainly would include a proposal to end the special tax treatment of capital gains, which was a major campaign promise in 1976.

The capital gains provision has been a primary target of "tax reform" advocates for the past several years. A capital gain is the profit from sale of securities or other assets. At present, only half a capital gain is subject to tax.

Ullman spoke with the President by telephone last week and urged him to slow overhaul of the tax code. The chairman said he was fearful that "we will be unable to digest" a fullblown tax overhaul plan.