Americans support the Treasury Department's income tax simplification and revision plan by 2 to 1, according to a nationwide Washington Post-ABC News public opinion poll.

The poll suggests that much of the support comes from people who have little or no understanding of Treasury's modified flat tax plan but think that the tax system is so unjust that they would accept any changes aimed at making it fairer.

Dissatisfaction with the income tax system is so strong that, while a majority of the people interviewed say they think "taxes are just too high," four of every five say they would not complain about the amount they must pay if they thought the wealthy were paying a fair share.

Almost all the opposition to the plan, announced by Treasury Secretary Donald T. Regan in November, comes from people who say they think they would pay more in income taxes under it. But even among that group, one-third support the proposal.

Among people who say they think that they would pay less or the same amount in taxes, eight of every nine back the proposal.

Just over half the 1,505 people interviewed by The Post and ABC News said they had heard or read of the Treasury proposal. Among them, 63 percent favored it and 27 percent opposed it.

Of all those interviewed, including those unfamiliar with the proposal, 53 percent said they favor it, 26 percent said they opposed it and 21 percent expressed no opinion.

A minority, whether or not they had heard of the plan, said they think that it will be enacted by Congress without substantial changes.

The Treasury proposal would replace the present 14 tax rates with three, of 15 percent, 25 percent and 35 percent, depending on income. According to Regan, 80 percent of all taxpayers would pay less or the same amount in taxes.

Overall, the Treasury tax program would yield the same revenues as would current law, Regan said.

The main change would be the elimination of most deductions and exemptions, on the grounds that people of equal means should pay about the same amount in taxes and that investments by individuals and businesses should be made because they are considered worthwhile rather than because they would lead to tax reductions.

Regan's proposal would nominally lower the tax rate for corporations but in reality shift some of the burden from individuals to corporations.

Interest groups ranging from the real estate industry to charitable organizations have attacked aspects of the plan as harmful to them. The proposal, however, also has drawn praise from some tax specialists and citizens' groups, among others.

Congressional leaders, while agreeing that something should be done about tax equity, generally have been lukewarm about the plan. Senate Majority Leader Robert J. Dole (R-Kan.) said Thursday that the chance for action on tax simplification this year "is not so good."

As an eventual proposition, Dole said, it is "very much alive."

In the Post-ABC News poll, conducted Jan. 11 to Jan. 16, 34 percent of the people interviewed who had heard or read about the Treasury proposal said they think that they would pay more taxes if it were enacted, 14 percent said they would pay less and 44 percent said they would pay about the same amount, with 8 percent expressing no opinion.

About 40 percent of those interviewed with incomes of $20,000 a year or less said they think that they would have to pay more in taxes, although the Regan proposal calls for elimination of income taxes for people below the poverty level and for tax increases for about only 10 to 15 percent of families with incomes of $20,000 or less.

This shows that many people who have heard of the Treasury tax proposal seem to be unfamiliar with its content. Other findings suggest strongly that the core of support for the Regan plan comes from public distaste for the current tax system. Asked whether they agree or disagree that "the present tax system benefits the rich and is unfair to the ordinary working man or woman," 72 percent said they agree.

On "whether nearly everyone who has the chance cheats somewhat on income taxes," 52 percent said they agree.

However, asked whether "it's no big deal" to underpay a little because "a lot of rich people pay no taxes at all," 24 percent agreed.

It seems, therefore, that the majority of Americans find tax cheating repugnant but feel that most people are cheating, quite possibly because they would think that they were being taken advantage of if they did not cheat.

On the question of whether "taxes are just too high," 61 percent agreed.

But the level of taxation seems less of a problem to most Americans than do inequities in the system. Eighty percent of those interviewed, and 84 percent of those who said taxes are too high, agreed that "I wouldn't complain about the amount I pay in taxes if I thought the rich were paying their fair share.