The Senate, after 11 days of work, last night virtually completed action on President Reagan's tax-cut bill, but Democrats kept the Republican majority from polishing it off before the critical tax test in the House today.

The Senate bill, which would be the largest tax cut in history, would cut individual rates by 25 percent across the board over three years, then establish what would amount to a system of permanent tax cuts by indexing rates to keep taxes from rising as wages increase with inflation.

Bidding for votes against a rival Democratic bill, the Republicans also sweetened their version with a variety of additional tax cuts for specific groups.

The Democrats in the House have done the same. These added forms of forgiveness include a reduction in the so-called marriage penalty, under which two workers pay more if married than if single; an increase in the existing child credit; the near elimination of the estate tax; a new kind of savings certificate sought by the savings and loan industry on which individuals could earn up to $1,000 and married couples up to $2,000 in tax-exempt interest each year; an increase to $1,500 to $2,000 in the amount that can be deducted and set aside each year in individual retirement accounts, and a provision under which all taxpayers, not just those who itemize, could claim deductions for charitable gifts.

The GOP bill also would significantly lower business taxes, mainly through a new system of accelerated depreciation write-offs that would save companies $55.3 billion through fiscal 1984.

These are added breaks for the oil industry, some $6.5 billion in windfall profits tax cuts over five years. Similarly, the trucking industry would be saved $356 million over six years to make up for lost revenues resulting from trucking de-regulation legislation passed in the last Congress. r

In the House, meanwhile, the key floor flight took final form as the Rules Committee established ground rules for what will be essentially three votes.

The first will be on an alternative proposed by liberal Democrats calling for a one-year tax cut without any of the further adornments in the other bills. It is almost sure to fail.

The next and most important vote will be on the Republican alternative, a bill very similar to the Senate bill, although it has even more special-interest amendments, including tax breaks for the oil industry of $13 billion to $16 billion, depending on the estimate.

If that fails, there will be a final vote on the Democratic bill.

The House Democratic leadership won a key test in a closed caucus when a request by liberal members of the party for a separate vote on the oil amendments in both the GOP and Democratic bills was defeated.

Rep. Barney Frank (D-Mass.) moved to make the anti-oil amendment in order. He was opposed by House Speaker Thomas P. (Tip) O'Neil Jr. (D-Mass.), who successfully moved to table his proposal. O'Neill was supported by Rep. Dan Rostenkowski (D-Ill.), chairman of the Ways and Means Committee.

Rostenkowski told his Democratic colleagues that it is essential for the party to have one victory after the humiliating defeats at the president's hands in the two major fights so far this year over budget cuts. Though they have an ostensible majority, the Democrats' control of the House is tenuous at best.

The vote today will be the third major test of power of O'Neill, who has yet to be able to muster a majority in a direct confrontation with the president. At midday yesterday, O'Neill claimed to be holding his majority on the tax bill, but defections later in the day cast doubts on confident predictions of winning by a 10-to-20-vote margin.

The Senate bill, on which there will not be a final vote until the House completes action later today, contains the following major provisions.

Individual tax rates would be cut by a total of 25 percent, 5 percent on October 1, 10 percent on July 1, 1982, and 10 percent again on July 1, 1983; cuts would be the same percentage in each tax bracket.

Republicans argue that this is the fairest way to distribute cuts in proportion to the amounts taxpayers must pay, although Democrats contend that "bracket creep" through inflation and increasing Social Security taxes would leave many taxpayers in the lower brackets paying higher rates by the end of 1983.

In a basic restructuring of the tax system, the Senate bill, along with the GPO House counterpart, would index individual brackets, the $1,000 personal exemption and the zero-bracket amount (or standard deduction) to the consumer price index in 1985 and thereafter.

The minimum tax rate on so-called unearned or dividend and interest income would drop from 70 percent to 50 percent on Jan. 1, 1982. The capital gains rate would drop from 28 to 20 percent effective retroactively to June 10, 1981.

The marriage penalty would be reduced by permitting a deduction of 5 percent of the income of the lesser earning spouse up to a maximum of $1,500 in 1982, growing to 10 percent in 1983 up to a maximum deduction of $3,000.

Users of the short form when paying income tax would be able to take a deduction for charitable contributions in addition to the standard deduction or zero-bracket amount. By 1986, there would be no ceiling on the deduction, but it would be phased out of the law and require reenactment in 1987 if it were to continue.

The windfall profits tax rate on newly discovered oil would drop by 1986 from the current level of 30 percent to 15 percent. Oil royalty owners would get a $2,500 tax credit from the tax.

An increase in the child-care credit that would, for the first time, create a direct federal payment for lower income working couples who pay child care costs. As the Senate approved it yesterday, the credit would vary from 30 to 20 percent of child care costs, depending on income.

The maximum credit would be $720 for one child and $1,440 for two or more children. This would phase down to a maximum of $960 for families making $30,000 a year or more with two or more children.

The estate tax would be almost eliminated by raising the exempt amount from $175,000 to $600,000 by 1987, and dropping the tax entirely for bequests to surviving spouses.