The Senate Finance Committee's rewrite of the nation's tax code may be drawing considerable enthusiasm in Washington but it has failed to generate much in the way of either support or opposition nationwide, according to a new Washington Post-ABC News public opinion poll.

Two of every three people are reserving judgment, according to the survey, one of the first public polls to measure views on the pending legislation. In all, 22 percent said they approve of the plan, and 11 percent disapprove.

The survey was conducted from last Thursday to Monday evening.

The Finance Committee's wide-ranging tax revisions, approved unanimously on May 7, have drawn strong support from President Reagan and most Republican and Democratic leaders in Washington.

But only 17 percent of those interviewed said they understand most of the plan. Thirty-eight percent said they understand some of it, and 40 percent said they understand very little of it.

In each of these groups, support outweighs opposition by about 2 to 1.

Despite the public's lack of knowledge of the proposals, two factors -- perceptions of fairness and tax cuts -- emerge from the survey as the keys to how the final product will be received.

Among those who think the bill would make the tax system fairer, 35 percent approve the legislation and only 4 percent oppose it. Among those who think the changes will not make for a fairer tax system, 8 percent approve and 22 percent disapprove.

Among those who think their own taxes would be cut under the bill, 47 percent approve it and 6 percent disapprove. Only one person in six, however, expects to pay less in taxes under the Finance Committee plan. Twice as many think their taxes would go up, and among them only 9 percent support the legislation.

Under the proposal, the top tax rate for individuals would be 27 percent, compared to a current rate of 50 percent. Six million low-income people would be freed from any income tax obligation, and moderate-income taxpayers would pay at a 15 percent rate. Taxes for businesses would be reduced from a current maximum of 46 percent to a new maximum of 33 percent.

The loss of revenues would be made up by eliminating many current deductions, including, for people enrolled in pension plans where they work, deposits into Individual Retirement Accounts (IRAs). In the poll, only 30 percent approved eliminating such IRAs and 63 percent opposed the change.

The survey confirms that upper income people tend to make use of IRAs much more than people of moderate means, a factor that has been used as a rationale for curtailing IRAs. Overall, more than one in three people interviewed said they have contributed to an IRA in the past six years, including 28 percent who said they had an IRA last year.

Among people with annual household incomes of more than $50,000, seven in 10 of those who were interviewed have contributed to IRAs over the life of the program, and six in 10 contributed last year.

By contrast, among people with incomes of less than $30,000 in the poll, slightly more than two in 10 have contributed to IRAs in the past, and fewer than two in 10 had an IRA last year.

Despite that discrepancy, a majority in all income groups supported keeping IRAs as a deduction for everyone. And even among people who have never had an IRA, a majority wanted no tinkering with the deduction.