The House Ways and Means Committee, in its first moves to toughen President Carter's proposed "windfall profits" tax on oil, yesterday approved two key amendments that would enlarge the measure's tax bite by $1.9 billion over five years.

The action was seen as a sign that Democrats were likely to succeed in efforts to stiffen the tax. Yesterday's balloting pitted liberals and main-stream Democrats against oil-state congressmen and Republicans.

The panel first rejected, 21 to 14, a GOP motion that would have endorsed Carter's bill unchanged. The measure was regarded as a political gesture designed to embarrass the Democrats, who view Carter's plan as too mild.

The committee is to meet into late evening today and tomorrow in an attempt to complete work on the "windfall profits" legislation Thursday. Panel leaders hope to push the tax proposal through the full House before the July 4 recess.

Meanwhile, the Treasury formally unveiled details of its companion proposal to limit the oil companies' use of the existing foreign tax credit, with new estimates showing the plan would reduce the industry's tax breaks by $514 million this year and up to $914 million by 1985.

The legislation was sent to Capitol Hill at the request of Rep. Al Ullman (D.-Ore), the Ways and Means chairman, in preparation for hearings on the issue next week. Ullman pledged the session as a gesture to liberals.

The amendments the panel approved yesterday would stiffen the president's proposal first by postponing the date by which a key portion of the tax would be phased out, and second by increasing the rate at which so-called "marginal" wells would be taxed.

The first proposal, sponsored by Rep. Joseph L. Fisher (D-Va.), would belay the phaseout of the tax on "old" oil - essentially oil discovered before 1972 - to July 1984 rather than May 1983, as Carter had proposed. The measure would raise the bill's overall tax bite $417 million by 1982.

The second proposal, drafted by Rep. Richard Gephardt (D-Mo.), would tax oil from "marginal" wells - deep wells that are difficult to exploit - at the stiffer "old" oil rate rather than as "new" oil, as Carter's plan would do. That would glean an extra $13 million to $477 million.

The Fisher measure was approved 19 to 17. Gephardt's proposal carried on a vote of 22 to 14.

The two proposals together wold add a cumulative $1.9 billion to the $20.6 billion in a new revenues that the Carter proposal now is estimated to bring in. Revised estimates by the Joint Committee on Taxation show Carter's plan would net $1.5 billion to $5.5 billion a year between now and 1984.

The details of the Treasury's proposal to tighten the way oil companies can use the foreign tax credit essentially were in line with the administration's previous description of the plan.

The proposal sought to close what the Treasury asserted were "some evident loopholes" in the present law, first by requiring the firms to take account of their losses in figuring their tax credits and then by "recapturing" missed taxes if the companies later turn a profit.

The administration also wants to limit the use oil and gas extraction credits to offset tax liability on income from that phase of the companies' operations. At present, the firms may use these credits to reduce U.S. taxes on earnings from shipping, refining and marketing.

Proponents of the Treasury action say the limits are necessary because oil company tax credits are inflated by tax systems in oil-producing foreign nations, which count royalty payments as taxes to give the oil companies a larger write-off in the United States.

However, some liberals on the panel want to go even further than the administration, seeking to require that these royalty payments be counted as deductible business expenses rather than taxes, or even to deny the industry the use of foreign tax credits altogether.