In numbers that show virtually no change since September, voters are supporting Democratic candidates for Congress over Republicans by 59 to 41 percent nationwide, according to a new Washington Post-ABC News poll.

The poll also shows Democrats making gains in some of the most tightly contested congressional races, according to samplings of voter intentions in 37 districts.

Tuesday's elections will be a referendum on President Reagan and his handling of the economy, this latest poll suggests. Reagan's recent campaigning appears to have prompted some voters to move toward Republican candidates. But by the middle of last week, when polling was completed, it seemed to have moved an equal number into the Democratic column.

More than 1 in 5 of the likely voters interviewed cited the nation's high unemployment figures as the main factor in their decision, and those voters were supporting Democratic candidates by 4 to 1.

Another 1 in 5 focused on candidates' individual attributes, such as personality, overall ability or leadership. The tendency among them also was to support Democrats, but by a narrower 5-to-3 ratio. Only a handful volunteered that such highly publicized issues as the nuclear freeze or abortion legislation were motivating factors.

In the new survey, conducted Monday through Wednesday, the Post and ABC reinterviewed 2,262 people from among 3,198 registered voters who had been surveyed early in October. Some of the key findings:

* Relatively few likely voters, about 13 percent, appear to have been swayed by recent events or campaign advertising to the point where they have changed their intentions. Roughly equal numbers have switched in each direction, so that neither party shows any gain.

* Among likely voters, only 2 percent say they have not yet decided on a candidate, suggesting that last-minute campaign efforts will have to be strong to have an impact.

* The Democratic and Republican rank and file are somewhat more likely than they were earlier in the month to bolt and vote for candidates of the other party. In a break from the historical pattern, Republicans are marginally more likely to say they will make such a jump than are Democrats.

* Independents, a swing group that could make up about 30 percent of the electorate, have moved slightly toward the Democrats since the first poll. Independents who appear likely to vote now favor Democrats by 52 to 48 percent. At the beginning of the month they leaned 51 to 49 toward the Republicans.

* Democrats hold strong leads in all regions except for the West, where Republicans are ahead by 10 points.

The new poll does not attempt to forecast how many seats the Democrats may be expected to gain. In recent days, political party leaders and other observers, working with similar poll data, have issued estimates ranging from about a dozen to 40 or more. Some observers maintain that a 25-seat Republican loss would be critical in that it could destroy Reagan's working majority in the House.

If Democrats are to make such sweeping gains, it stands to reason that they must win the lion's share of the most closely contested races. The new poll offers findings that may be read either way, suggesting a range from substantial to only limited Democratic pick-ups.

Among the 37 congressional districts where the Post and ABC did special interviewing, eight are seats in which Democratic incumbents are running, 19 are seats in which Republicans are seeking reelection and 10 are open seats, half newly drawn.

Democrats are leading in seven of the eight races for the seats they now hold, just as they were earlier in the month. But they also hold leads in 12 of the 19 Republican districts, where in the first round of polling they were ahead in eight. In several of these districts, Democrats jumped ahead in part because of movement by independents who had previously leaned toward the Republicans.

If the poll findings hold through the election, then in 1983 those 27 seats will present a mirror image of their current standing: 19 will be held by Democrats and eight by Republicans for a pickup of 11 by the Democrats in these few districts alone. That could suggest the makings of a landslide.

But results in the 10 open seats suggest the opposite, with the Democrats going back a step from the earlier survey, when they led in five and the Republicans in five. In the new poll, Democrats are ahead in four and Republicans in six.