(CNN) -- Candidates are making their closing arguments as they try to win over New Hampshire voters, but the results of the first-in-the-nation primaries could come down to one thing: who can woo the Independents.
Sen. John McCain won the GOP New Hampshire primary in 2000.
Independent voters make up about 40 percent of the Granite State's electorate. Registered party members can vote only in their own party's primaries, but Independents can vote in either.
Independent voters changed the GOP race during New Hampshire's 2000 primary. Although George Bush beat Sen. John McCain among registered Republicans, independents, who accounted for 41 percent of the total GOP turnout, handed McCain the win.
Independents constituted 45 percent of Democratic primary voters in 2004.
With independents outnumbering both Democrats and Republicans, a strong backing from them could propel anyone's game in the unpredictable state. Watch how candidates are trying to sway undecided voters »
New Hampshire, once known for its rock solid Republican ground, supported GOP presidential candidates in 10 of the 11 presidential elections between 1948 and 1988. The 1990s saw a shift, with Bill Clinton carrying the state twice.
In 2000, George W. Bush took New Hampshire by one percentage point, but four years later the state supported John Kerry.
At the same time, Democratic challenger John Lynch ousted Republican Gov. Craig Benson. Lynch was re-elected with 74 percent of the vote in 2006.
Region by region
New Hampshire's rural North is traditionally Republican, with the exception of Berlin.
The South is another traditionally Republican region, but the area, comprised of small towns and suburbs, receives many new residents from neighboring Massachusetts. These residents, escaping high taxes, infuse the area with their Democratic politics and help make New Hampshire a swing state.
Concord, the state's capital, and the Connecticut Valley represent the state's most Democratic area.
New Hampshire's industrial area, more Democratic than the state as a whole, includes diversified Manchester and Nashua.
On the Republican side, McCain has received the support of almost every major paper in the state, including the Union Leader, an influential conservative newspaper.
The endorsement is among the most coveted and sought after among candidates in the GOP field.
McCain picked up the support of the Nashua Telegraph and the Concord Monitor, along with more than 20 other newspaper endorsements in the state.
CNN's Robert Yoon and Alan Silverleib contributed to this report.