Democrats were poised to boost their thin US Senate majority early on Wednesday, taking over hotly contested Republican seats in Massachusetts and Indiana while holding on to most of those they already had, including in Virginia and Missouri.

While the result was no surprise, Republicans had given themselves an even chance of winning a majority, so the night represented a disappointment for them, especially for the Senate's Republican leader, Mitch McConnell, thwarted in his twin missions of taking over the Senate and defeating President Barack Obama.

Overall, Democrats were poised to boost their majority from 53 to 55, counting two independents who vote with the party in the Senate.

The Republican setback was in part self-inflicted, the result of internal battles waged in the party.

Had conservative Republican Richard Mourdock not defeated veteran moderate Richard Lugar in Indiana's primary, for example, that seat might have stayed in Republican hands instead of being won by Democrat Joe Donnelly on Tuesday.

Missouri Senator Claire McCaskill, who kept her seat on Tuesday, had been considered a vulnerable Democrat until conservative Todd Akin won the state's Republican primary.

Akin and Mourdock stumbled badly with remarks about abortion that were widely criticized as unsympathetic to rape victims, handing Senate Democrats two of Tuesday's biggest wins.

The election left the Senate somewhat more polarized, with generally fewer moderates and more conservatives among the Republicans and more liberals among the Democrats.

Republicans remained firmly in control of the House of Representatives, ensuring that Congress still faces a deep partisan divide as it turns to the year-end "fiscal cliff" that threatens to crush US economic growth.

With President Barack Obama retaining the White House, the status quo result portends more partisan gridlock.

"That means the same dynamic. That means the same people who couldn't figure out how to cut deals for the past three years," said Ethan Siegel, an analyst who tracks Washington politics for institutional investors.

It was a rough night for the some of the conservative Tea Party's loudest and most controversial members.

The movement's leader, failed Republican presidential candidate Michele Bachmann, was fighting to keep her congressional seat in Minnesota in a race that was too close to call.

Representative Joe Walsh, another Tea Party activist, soundly lost his bid for a second term representing a district outside of Chicago, and Representative Allen West was losing to his opponent in Florida, Democrat Patrick Murphy. But by early morning, the Miami Herald was reporting that outcome was too close to call.