Polls indicate Republican Scott Brown has the lead over Democrat Martha Coakley. At stake is the Democrats' supermajority of 60 seats in the Senate.
Kennedy died Aug. 25 and Paul Kirk was selected to represent the commonwealth until the special election, which was predicted to be an easy win for Democrats.
But a series of events have put the outcome in doubt. Even in Massachusetts there is a general dislike for many national Democratic Party policies and Coakley has run a poor campaign.
High-profile Democrats, including former President Bill Clinton and current President Barack Obama last weekend, have campaigned to keep the seat in their party's hands.
Brown has run a campaign from the beginning that didn't concede anything to Coakley. That helped him gain momentum, which then grabbed national attention -- and donations -- further fueling his efforts.
Brown said he would be the needed vote for Republicans to stop the Democrats' healthcare reform legislation. It is unclear, how long it will take state officials to confirm any vote outcome either way.