Democrats have made improvements in poll numbers regarding the upcoming U.S. midterm elections, but still lag behind their Republican opponents, a new Washington Post-ABC News poll showed on Tuesday.

Among likely voters, 49 percent said they will support Republican candidates on Nov. 2, while 43 percent said they will cast their ballots for Democrats. The six-percentage advantage enjoyed by Republicans is about half of that in early September.

The poll also finds that Republican candidates hold a significant edge in preferences for the midterm elections among likely independent voters, with a margin of 53 percent to 33 percent.

The preferences of independents, the largest group of voters in the United States, are key to the results of the midterm elections.

Thirty-seven percent of registered voters are independents or other non-partisans, while 34 percent are Democrats and 29 percent are Republicans, according to data from the Pew Research Center.

Republicans are predicted to have a good chance to recapture control of the House of Representatives and gain some seats in the Senate, if not a majority. But as recent polls show Democrats are catching up in the races, congressional Republican leaders have expressed cautions to their party members.

"I think that it's been predicted at this point that the poll numbers will tighten up," House Minority Whip Eric Cantor said Monday during an appearance on CNBC, warning that there is no reason for Republicans to be complacent.

He said that any tightening-up in the race for control of Congress might be due to Obama and Democrats' increased focus on energizing their own base.