With Democrats who previously supported Ronald Reagan now returning to the fold, voters across the nation appear to be lining up in party-line formation for the congres- sional elections six weeks from now, according to the findings of a Washington Post-ABC News poll.

The result is a lead of 60 to 40 for the Democratic Party among likely voters, with the Republicans trailing substantially in all regions of the country except the West, where they are ahead by a 6-point margin. In all, 92 percent of Democrats say they would vote for Democratic candidates for Congress, while 90 percent of rank-and-file Republicans are backing their party's nominees. Independent voters, the likely key to many of the 435 House races, are sharply divided but leaning toward the Democrats by 50 to 40 percent, with 10 percent uncertain.

The poll strongly suggests that two tightly linked factors are dominating voters' thinking: perceptions of Reagan and the health of the nation's economy. Voters who approve of Reagan's performance as president and who feel the economy has seen its worst days are solidly in the Republican column. But only 40 percent of all voters subscribe to both those views.

The change among Democrats since the Republican Party's sharp gains in 1980 has been dramatic. Reagan was carried to his easy victory over Jimmy Carter by drawing support from roughly 25 percent of the Democratic voters, according to election day exit polls. These disenchanted Democrats also helped Republicans take control of the Senate and make inroads in the House of Representatives. But now, with the 1982 campaign just getting under way, the new poll shows that Republican congressional candidates are receiving support from only 6 percent of the Democrats.

Three-quarters of the Democrats who say they voted for Reagan told Post-ABC News interviewers they now intend to vote for Democrats for Congress. The only Democratic voters who tend to support Republican candidates in any numbers at all are the relatively few who both approve Reagan's handling of the presidency and feel that the economy is improving.

Political scientists have differing views as to how important the popularity of a sitting president may be in the outcome of off-year elections. But the new poll, along with earlier findings by the Gallup organization for Newsweek magazine and other Washington Post polls, indicate that feelings about Reagan may be crucial in many election campaigns this November.

The Gallup poll compared the "Reagan effect" with that of Carter four years earlier. Among those who disapproved of Carter in 1978, slightly more than one-third said they would vote for Republicans for Congress that year. But among those who disapproved of Reagan early this year, more than three-quarters said they would vote for Democrats this fall.

Similarly, in polling before the Virginia gubernatorial election last year, The Washington Post found views on Reagan and his policies to be the key in determining how individuals would vote.

Should Democrats vote as strongly along party lines as the new poll suggests, Republicans in many congressional districts will have to win over large majorities of independent voters to avoid losing in November. Right now that does not appear to be happening.

The link among Reagan, the economy and voting intentions is nowhere clearer than among political independents. Half the likely independent voters interviewed by The Post and ABC News feel the recession has bottomed out and express approval of Reagan. Among them, Republican candidates are running well ahead, by 62 to 26 percent. But the other half either disapprove of Reagan or see no improvement in the economy, and they are choosing Democratic candidates for Congress over Republicans by 77 to 18 percent.

Because the congressional elections consist of 435 separate campaigns for seats in the House, it is difficult to convert the overall 60-40 Democratic lead into a number of seats the Democrats may gain. Working with a similar but slightly smaller margin last week, the Gallup Poll said Democrats stand to pick up 30 seats.

Other political analysts, however, see very few hotly contested races and, as of now, expect the Democrats to gain between six and 18 seats. Currently the Democrats have 242 members in the House and the Republicans 192 with one vacancy.

Other findings in the poll, conducted by telephone from Sept. 9 through 13, include:

* The so-called "gender gap," or the difference in voting preference between men and women, may continue in a limited manner through the November elections. Women, according to the poll, will outnumber men in total vote and lean slightly more than men toward Democratic candidates.

* The Democratic Party is widely perceived as better able than the Republicans to cope with such problems as unemployment, fair taxation and protection of the Social Security system. The Republican Party is seen as slightly better at controlling government spending and inflation. On all these issues, the GOP has lost ground to the Democrats, often sharply, since November, a comparison of Post-ABC News polls shows.

* People who say they support the positions of the religiously fundamentalist and politically conservative Moral Majority, are much more likely to support Republican candidates than are other voters. But only 10 percent in the poll say they support Moral Majority positions. Twenty percent say they oppose the group and the rest say they are not familiar with it -- a sharp decline since 1980, when 50 percent in a Post-ABC News poll said they knew of the Moral Majority.

In all, 1,505 people were interviewed. Among them, 500 were judged to be almost certain to vote in November because of the interest they expressed and their statements about past voting. The figures in this article are based on those 500 interviews, but there is little overall difference between them and the total population interviewed on almost all issues, including voting preference.