The sweeping reelection victory that President Reagan appears headed for on Tuesday is being built on broad support in all regions and on the failure of Democrat Walter F. Mondale to recapture wavering Democrats in the final weeks of the campaign, according to a new Washington Post-ABC News opinion poll of almost 9,000 registered voters.

With an 18-point nationwide lead in The Post-ABC News poll -- a lead that converts to about 20 points when undecided voters are taken into account -- and with similar leads in other major national surveys, Reagan could roughly double the margin of his triumph over President Jimmy Carter four years ago.

In 1980, Reagan carried all but five states, but his margin of victory in many was narrow. This year Mondale appears close to Reagan in only a handful of states, with the District of Columbia his only certain winner.

The difference this year is that Reagan appears to have increased his vote-getting ability among almost every large group in the electorate.

Among men, Reagan won about 55 percent of the vote in 1980, according to network exit polls. The Post-ABC News survey shows he is getting 60 percent or more of the male vote this time.

Among women, Reagan won less than 50 percent in the 1980 election; he may win as much as 55 percent of the female vote this year. The gender gap, which first came to public attention after the 1980 campaign, still exists. The president is doing much better among men than women. But with a majority of women supporting Reagan, the gender gap is not likely to affect the outcome.

In 1980, about 85 percent of the Republican vote, about 55 percent of the independents and 25 percent of the Democrats backed Reagan over Carter and independent candidate John B. Anderson. This time, Reagan appears to have increased his strength among all three groups. The Post-ABC News 48-state survey, conducted Monday through Thursday of last week, shows Reagan winning about 90 percent of the Republicans, 65 percent of the independents and about 29 percent of the Democrats.

Reagan's gains appear sharpest among two key groups: young voters aged 18 to 29 and Catholics. In 1980, Reagan did worst among younger voters, drawing support from about 45 percent of them. This year, he is doing best among them, drawing support from more than 60 percent of the 18-to-29-year-olds in the survey.

Reagan won a bare majority of Catholics, 51 percent, in 1980. The new poll shows him supported by more than 60 percent of the Catholics interviewed, despite the fact that Mondale's running mate, Geraldine A. Ferraro, is Roman Catholic.

Of the large voting blocs, only blacks seem likely to give Reagan less support in 1984 than in 1980. And even among them, there seems to be no falloff for the president. He drew support from about 14 percent of the blacks in 1980, and the poll suggests he will get about the same proportion of the black vote on Tuesday.

In last week's survey, The Post and ABC News conducted what is known as a panel-back, attempting to re-interview 11,807 registered voters who were first polled from Sept. 22 to Oct. 2. A total of 8,969, 76 percent, were re-interviewed.

In the original poll, Reagan led Mondale by 55 to 37 percent nationwide, with 8 percent undecided. Much of the strongest campaigning of 1984 took place between the two surveys, including two televised debates between the contenders and one between Vice President Bush and Ferraro.

Interim Post-ABC News surveys and other polls showed the gap beginning to close after the first Reagan-Mondale debate on Oct. 7. Mondale was within 12 points of Reagan in a Post-ABC News poll and within 9, according to a Louis Harris poll, after the first debate. Other surveys showed somewhat wider margins but the same trend.

At that stage, the Mondale strategists set their hopes on reclaiming large numbers of Democrats who were supporting Reagan but who indicated that they might be wavering. The panel-back survey suggested that large numbers of Democrats, about 7 million, did change their minds during October, but that almost as many switched toward Reagan as away from him.

By contrast, relatively few Republicans -- about 2 million -- may have switched over that time, according to the poll, with Mondale making a small net gain among them. But that was canceled out by small shifts toward Reagan among independent voters.

In all, about 10 percent of all registered voters changed during the past month, according to the poll, with some being previously undecided voters making up their minds. That suggests little volatility compared to recent elections.

NOTE: A total of 8,969 registered voters were interviewed by telephone Monday through Thursday. The survey, the second stage in one of the largest preelection polls ever conducted, consisted of re-interviews with voters who were among 11,807 polled from Sept. 22 to Oct. 2. In the initial round, the survey yielded samples varying from 150 interviews in the smaller states to 800 in states with the most electoral votes. In the second round, the smallest number of interviews in any state was 105, the largest, 582. Nationally, the poll is subject to a theoretical margin of sampling error of about 3 percentage points. The margin is higher on the state level.