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.
CANDIDATES Scott Brown; Elizabeth Warren
Brown: U.S. Senate, 2010-present; Massachusetts state Senate, 2004-2010; state House of Representatives, 1998-2004; Board of Selectmen in Wrentham, Mass., 1995; assessor of Wrentham, 1992.
Warren: Has never held elective office.
Brown: Saying that his "model" Supreme Court justice is ultra-conservative Antonin Scalia.
Warren: Handling of her controversial claim of Native American ancestry.
2012 MONEY RACE
Brown: $24.4 million raised, $21.3 million spent as of Sept. 30 (latest available data). Roughly 16 percent was from small contributions, 67 percent from large contributions. Ten percent came from political action committees.
Warren: $28.2 million raised, $15.9 million spent as of Aug. 17. Roughly 43 percent came from small donors, 55 percent from large contributors. PAC contributions amounted to 2 percent of the money raised.
Brown: Describes himself as pro-choice, but opposes what critics call "partial birth" abortion and supports laws that require parental consent.
Warren: Staunch supporter of abortion rights. Has vowed not to vote for any Supreme Court nominee who opposes abortion.
Brown: Supports the right to carry a concealed firearm, but thinks licenses and background checks should be required.
Warren: Supports background checks to purchase firearms; also supports reinstating the federal ban on assault weapons.
Brown: Opposes the DREAM Act, which he called "a form of back-door amnesty." Supports tighter border control and an overhaul of U.S. immigration laws. Says "government should not pass laws that encourage illegal immigration."
Warren: Says the DREAM Act is the "right" thing to do. Supports an overhaul of U.S. immigration laws.
Brown: Has voted to repeal the Affordable Care Act of 2010
Warren: Supports the Affordable Care Act.
Brown: A Congressional Quarterly study concluded that Brown was the second-most bipartisan senator in 2011.
Warren: Has a history of working with Democrats and Republicans in Washington.
Brown: Signed the Grover Norquist-inspired pledge not to raise taxes under any circumstances. Voted against a Democratic proposal that would have allowed tax cuts on the wealthiest Americans to expire.
Warren: Opposes any tax increase on the middle class. Supports the "Buffett Rule" to apply a minimum tax rate of 30 percent to the wealthiest Americans.
Brown: Married to former local TV reporter Gail Huff in 1986. The couple have two daughters, Ayla and Arianna.
Warren: Has two children, Amelia and son Alex, from her marriage to NASA mathematician Jim Warren, whom she later divorced. Married Harvard law professor Bruce Mann in 1980.
Brown: Christian Reformed Church in North America
Brown: Bachelor of arts from Tufts University, 1981. Law degree from Boston College Law School, 1985
Warren: Bachelor of arts from the University of Houston, 1970. Law degree from Rutgers School of Law-Newark, 1976.
Brown: He posed nude for Cosmopolitan magazine in 1982.
Warren: She was nine months pregnant when she graduated from law school.
Brown: 1979-present, Massachusetts Army National Guard. Currently holds rank of colonel. Part-time modeling through the 1980s.
Warren: Assistant to the President and Special Adviser to the Secretary of the Treasury on the Consumer Financial Protection Bureau, 2010-2011; chairwoman of the Congressional Oversight Panel, 2008-2010; chief adviser to the National Bankruptcy Review Commission, 1995-1997.