Editor's note: John P. Avlon is senior political columnist for The Daily Beast and author of the forthcoming "Wingnuts: How the Lunatic Fringe is Hijacking America."
(CNN) -- On Tuesday, the eyes of the political world will be turned to Massachusetts where an unexpectedly close special election is being held to determine who will succeed Ted Kennedy in the Senate.
Conventional wisdom labels Massachusetts a liberal bastion, but this stereotype misses the mark. In fact, there are more independents in Massachusetts than Democrats or Republicans.
Take a look at the numbers: There are roughly 2.1 million independent voters in Massachusetts, 1.5 million Democrats, and 500,000 Republicans. Yes, Democrats far outnumber Republicans in the Bay State -- especially in Boston -- but there are more independents than Democrats and Republicans combined.
This is the key to understanding why the race between Republican State Sen. Scott Brown and Massachusetts Attorney General Martha Coakley, a Democrat, is so close. It reflects a broader dynamic occurring in American politics: Independents are the largest and fastest growing segment of the electorate.
There are now 11 states like Massachusetts, where independents outnumber Democrats or Republicans outright. Winning independents is the key to winning any election -- and right now, independents are angry at the Democrat-controlled congress.
Independent voters have been swinging against President Obama's policies since the spring stimulus bill and the budget's unprecedented deficit spending. This gap has been compounded by questions over health care reform and the growth of government it has come to symbolize.
Independents voted for President Obama by an eight-point margin in '08. They responded to his post-partisan rhetoric and his promises to unite the nation. But the narrow partisan play-to-the-base agenda pursued by conservatives in the Bush administration now seems to be replicated by the liberal Democratic leadership in Congress.
It's provoked a backlash that reminds independent voters why they like the checks and balances of divided government: They dislike ideological arrogance and legislative overreach that comes when one party controls both the Congress and the White House.
Independents' preference for checks and balances also helps explain why Massachusetts Gov. Deval Patrick is the first Democrat to hold the post since Michael Dukakis.
The intervening two decades saw a steady stream of Republican governors beginning with centrist Republican Bill Weld, whose record as a law and order fiscal conservative combined with a liberal-to-libertarian approach on social issues fit independent voters' preferences perfectly.
Although you wouldn't know it for the conservative supporters rallying around his campaign, Scott Brown fit this model as well. Fitting the state he seeks to represent in the Senate, Brown is not a far-right social conservative.
He is, for example, pro-choice when it comes to abortion, saying "this decision should ultimately be made a woman in consultation with her doctor." But he is far from an absolutist on the issue, like most Americans. He is opposed to the partial-birth abortion, in favor of parental notification and wants to encourage adoptions.
If he wins the special election, Scott Brown will be the first Republican senator from the Bay State since Ed Brooke, the centrist Republican attorney general who became the first African-American popularly elected to the Senate back in 1966. But Republicans shouldn't be breaking out the champagne just yet.
While Brown has the momentum, a low turnout special election usually favors the local party machine and Democrats will be relying on their get-out-the-vote operation with an assist from the unions. Still, independents and Republicans may be the more motivated voters in this election cycle -- the Democrats' filibuster-proof 60-seat majority is hanging in the balance.
Whatever the outcome in this potential photo-finish Senate race, Republicans and Democrats should be learning the essential lesson that their play-to-the-base impulses try to ignore: Independent voters hold the balance of power in American politics. Even in Massachusetts.
The opinions expressed in this commentary are solely those of John P. Avlon.