Adjust font size:

WASHINGTON (CNN) -- The percentage of likely voters who plan to vote for Democrats in Tuesday's congressional elections increased in the past week, according to a CNN poll conducted during the weekend.

Those voters supporting Democrats also seem less likely to change their minds before casting ballots, the poll found

Democrats hold a 20 percentage-point advantage -- 58 percent to 38 percent -- over Republicans among likely voters in the survey released Monday morning. The Democratic advantage was 11 percentage points -- 53 percent to 42 percent -- in a poll a week ago.

Pollsters asked people who identified themselves as being likely to vote which party's candidate they would vote for in their congressional district if the election were being held today. (Watch how the parties are trying to get voters to the polls -- 2:13)

On this question, the poll has a sampling error of plus or minus 3 percentage points.

In the so-called "poll of polls," which averages the results of five national surveys, Democrats have a 53 percent to 41 percent margin over Republicans.

When asked if it is possible they could change their mind before Election Day, 17 percent of those likely voters who supported Republican congressional candidates said they might instead chose a Democrat, while 10 percent of those supporting Democrats said a switch was possible.

The poll's sampling error on this question was plus or minus 6 percentage points.

The CNN poll, conducted by telephone Friday through Sunday by Opinion Research Corp., interviewed 1,008 adult Americans, including 636 likely voters.

Democrats need a 15-seat pickup in Tuesday's elections to regain control of the House of Representatives and a gain of six seats to reclaim the Senate.