WASHINGTON (AFP) - A third of the seats in the US Senate are up for grabs in congressional elections on November 4, with Democrats expected to reap gains to expand their current 51-49 majority.Here is a summary of the top battlegrounds: VIRGINIA A seat vacated by retiring veteran Republican John Warner is an almost certain pickup for Democrats who are running popular former state governor Mark Warner, who is no relation to the incumbent. Latest polls show Warner leading conservative Republican Jim Gilmore by 30 percentage points. NEW MEXICO Republican Senator Pete Domenici, first elected in 1972, is retiring from what was once a rock solid safe seat for his party. But polls put Democrats on course for victory, with their candidate Tom Udall up by 15 percentage points on Republican Steve Pearce. COLORADO Republican Wayne Allard is another retiring senator, and latest polls show Democrat Mark Udall, a cousin of the Democratic candidate in Colorado, up by double figures. Democratic presidential candidate Barack Obama has made the state one of his key targets, and if he wins, could sweep Udall to victory on his coattails. NEW HAMPSHIRE Former governor Jeanne Shaheen, a powerplayer in Granite State politics for years, is running a strong race in her bid to unseat John Sununu, the Republican who beat her in a much stronger Republican year, 2002. A poll last week had Shaheen up 10 points. OREGON Republican Gordon Smith is in the fight of his life in this liberal far western state and has run advertisements lauding his work with prominent Democrats. Obama is set for a big win here, where early voting is already underway, which could be good news for Democrat Jeff Merkley. NORTH CAROLINA Republican Elizabeth Dole was never expected to have to fight for this seat, but she has been hammered for spending little time in the state and taking her position for granted. Recent polls have shown Dole polling less than 45 per cent against Democrat Kay Hagan. MINNESOTA Democratic hopes are rising as wise-cracking former comedian Al Franken is taking advantage of the shockwaves from the Wall Street meltdown to put the heat on Republican incumbent Norm Coleman. This is an unusual three-way race also featuring an independent candidate. GEORGIA Another Republican stronghold which is competitive, underlying the tough task the party has in a political year offering all the advantages to Democrats. Latest polls show incumbent Saxby Chambliss with a narrow single digit lead. ALASKA Veteran Republican Senator Ted Stevens should be a lock here, but was found guilty of corruption and his ratings plummeted. He looks set to lose his seat. KENTUCKY Only a political earthquake could oust Mitch McConnell and deprive the Senate minority leader of a fifth term, but Democrats are still dreaming big.