Washington - The Democrats have picked up four seats from Republicans in the Senate in elections on Tuesday and are poised to make significant gains to widening their majorities in both congressional houses, network projections showed.

The Democrats won two seats vacated by retiring Republicans in Virginia and New Mexico, while Democratic challengers ousted two of their Republicans opponents in New Hampshire and North Carolina.

Democrats were also hopeful about knocking off the Republican minority leader in the Senate, Mitch McConnell, an outcome that would be a major blow to Republicans. McConnell was clinging to a 51- 49 per cent lead against Democrat Bruce Lunsford with 65 per cent of votes counted. CNN predicted McConnell will hold onto the seat.

In Virginia, former Democratic governor Mark Warner defeated a former Republican governor, James Gilmore, to take the seat occupied by Republican Senator John Warner, who is retiring at the end of his term and is not related to his successor.

Democratic challenger Kay Hagan has defeated Republican Senator Elizabeth Dole in North Carolina, while John Sununu, a Republican senator, was ousted by Democratic challenger Jeanne Shaheen.

And in New Mexico, Democrat Tom Udall has the seat occupied by longest-serving Republican Pete Domenici, who is also retiring.

Heading into the election, the Democrats controlled the 100-seat Senate 51-49 and the House of Representatives with a 236 to 199 majority. Democrats were optimistic that President George W Bush's unpopularity would boost their numbers in Congress.

All 435 seats in the House are up for grabs, while about one- third of the Senate is standing for election.

Senator Joe Biden, Barack Obama's vice presidential candidate, handily won re-election in Delaware. If Obama wins the election, Biden would have to vacate his seat, but he would likely be replaced by a Democrat appointed by the state's governor.

Democrats were also hopeful of defeating Alaska Senator Ted Stevens, the longest serving Republican, following his conviction last week on corruption charges. (dpa)