By 2 to 1, Americans oppose President Reagan's plans to abandon the SALT II treaty and feel the United States should abide by it until another arms control agreement is reached, according to a new Washington Post-ABC News poll.

A large majority, 70 percent, take Reagan at his word when he says he truly wants a reduction in nuclear armaments. But there is a widespread belief, greater than in previous surveys, that the president has not done as much as he should to achieve that goal.

In addition, the survey, conducted from Thursday through Monday evening, shows the public remains opposed to military and other aid to the Nicaraguan contras, or counterrevolutionaries, with 62 percent against and 29 percent in favor. The House is scheduled to vote today on variations of Reagan's request for $100 million for the contras.

The survey's findings show the public basically believing that the United States should not attempt to help overthrow the Sandinista regime. Despite Reagan's continuing appeals, Republicans, Democrats and independents all opposed his request by wide margins.

The poll also found no groundswell of support for the sweeping tax code revision that was passed, 97 to 3, by the Senate yesterday. A majority continues to have no opinion. Only 22 percent of those interviewed favored the bill, the same as in a Post-ABC News poll last month, and 15 percent disapproved, a 4 percentage-point increase.

On SALT II, those questioned were asked whether they agreed with Reagan's position -- that he will not be bound by the unratified 1979 strategic arms limitation treaty but will seek a better one -- or with critics who say the United States should follow the treaty until a new accord with the Soviet is reached.

Overall, 61 percent sided with the critics and 29 percent with the Republican president. In his party, 53 percent opposed Reagan's position. Among Democrats, 70 percent sided with the critics. So did independents, 2 to 1.

Two of every three people admitted knowing "nothing at all" or "not much" about SALT II. Only three of every 100 claimed to know "a good deal" about it, and three of 10 said they knew "a fair amount."

On most issues related to the treaty and disarmament, however, the differences in outlook among these groups were slight. Those who claimed greater familiarity with the pact were somewhat more likely to believe it had made the world a safer place. Reagan announced that he would no longer be bound by SALT II about a month ago, and drew sharp criticism from some Republicans and Democrats on Capitol Hill. In recent days he has repeatedly said that he wants a better treaty, and that he thinks Soviet leader Mikhail Gorbachev also seriously desires arms control.

All segments -- Democrats, Republicans and independents -- give Reagan credit for wanting to make progress in arms control. But overall, 55 percent in the survey said he has not done enough to limit the buildup of nuclear weapons in the United States and the Soviet Union, compared to 36 percent who said he has done all he should.

Those figures show people more critical than in four polls dating back to March 1982. Most recently, just before his summit meeting with Gorbachev last November, Reagan held an advantage, 48 to 44.

Regarding tax code revision, despite the lavish praise the Senate bill has won from the president and from members of both parties in Congress, more than 60 percent of those surveyed said they did not know enough about the measure to judge it one way or the other.

People who said they understood most of the Senate bill -- 16 percent in the poll -- favored the measure by about 2 to 1. Support for the bill increased somewhat from last month's poll among people with household incomes of $50,000 a year or more, but opposition rose somewhat among those of more moderate means.; Polling assistant Kenneth E. John helped prepare this report.