The Massachusetts special elections have become a high-stakes national contest on Tuesday.
Voters streamed to the polls in a close election for a new U.S. senator that could derail Democrats' dominance in Washington and influence President Obama's health care reform.
High turnout could benefit the Republican candidate Scott Brown more, even though registered Democrats in Massachusetts hold a sizable numerical advantage.
Brown has promised to be the Republican's 41st vote to kill healthcare legislation.
Democratic Party icon Edward Kennedy, who held the seat for almost 47 years, died in August of brain cancer.
Democrat Paul Kirk was appointed by the state's governor in September to occupy Kennedy's seat and will remain in the Senate until a winner is sworn in.
Massachusetts last elected a Republican to the Senate in 1972, but the weak economy and doubts about the healthcare overhaul have moved voters to abandon political loyalties.
Democrats now control 60 votes in the Senate to 40 for the Republicans. The loss of one seat could hamper the Democrats' ability to cut off debate and proceed to a vote on the planned healthcare overhaul.
The Democratic candidate Martha Coakley has been criticized for a lackluster campaign. She took almost a week off from the campaign trail around Christmas, at a time when Brown's appeal was on the rise.
Millions of dollars have flooded into the state to buy nonstop television advertising for both sides, transforming a relatively sleepy contest into a bitter brawl.