A special election two years ago to fill the late Sen. Edward Kennedy's seat brought a huge change to Massachusetts politics: the first Republican U.S. senator in nearly four decades.
Not surprisingly, Sen. Scott Brown has been a thorn in the side of the state's Democratic establishment since his 2010 victory.
"Most of the time here, you have a Democratic primary and then shortly thereafter, a swearing in," says Maurice Cunningham, a political science professor at the University of Massachusetts, Boston.
But Brown, who previously served in the state Legislature, has cast himself as a bipartisan moderate — an image he needs to cultivate to survive in his dark blue state. His Democratic challenger, Elizabeth Warren, is a Harvard Law School professor whose work led to the creation of the U.S. Consumer Financial Protection Bureau.
The latest polls show Brown and Warren in a dead heat, even though Massachusetts is likely to swing heavily to President Obama at the top of the ticket.
A Brown win would be a huge boost for Republicans in the state who'd like to prove his 2010 victory was no fluke. If Warren wins, she would become the state's first-ever female U.S. senator.
The race has attracted massive amounts of cash, with Republican groups funneling money in hopes of holding onto the seat and Democrats just as determined to knock off Brown in their push to keep control of the U.S. Senate.
Here's a side-by-side comparison of Brown, 53, and Warren, 63.