Democrats lead in five of 10 crucial senate battlegrounds, including three Republican-held seats in Pennsylvania, Montana and Rhode Island and in Democratic-held Maryland and New Jersey, according to the Reuters/Zogby survey published on Thursday.
However, the poll shows Republican incumbents lead in Virginia and Missouri, and senate contests in Republican-held Ohio and Tennessee as deadlocked.
Joseph Lieberman, the Connecticut senator running as an independent, is revealed to have a big lead over Democrat Ned Lamont.
To gain a senate majority, Democrats must hold their own vulnerable seats and take six of the seven at-risk Republican seats, including removing five Republican incumbents, a difficult but not impossible task.
John Zogby, a pollster, said: "It looks like Democrats will make gains, but it will be very difficult for them to take control. It is going to take an awful lot of work for them to pick up six seats."
The polls show Lieberman, the 2000 Democratic vice presidential nominee who lost an August primary fight to Lamont, with a 20-point lead over his rival.
Other polls have given Lieberman a smaller lead in the high-profile race, which will not have a bearing on the Senate balance of power.
Lieberman has promised to vote with Democrats if he wins his race as an independent.
The polls, taken from September 25 to October 2 in 10 of the country's most competitive Senate races, surveyed at least 600 likely voters in each state. They have a margin of error of plus or minus 4 percentage points.
Public unhappiness with George Bush, the US president, the Iraq war and the direction of the country has created a difficult political environment for Republicans and given Democrats momentum in the election battle for the US congress.
The polls overlapped by three days with the unfolding sex scandal involving Florida Representative Mark Foley's explicit internet messages to teenage male congressional pages.