In theory, the US operates a multi-party political system. However in reality this is not the case. The Republican Party and the Democratic Party are the dominant political parties in the US, and this will remain the case in the 2008 election.
The Republican Party was founded in 1854 and grew to prominence with their first president Abraham Lincoln. The Republican Party is currently in power in the White House with George W. Bush as president.
The Democratic Party was founded by Thomas Jefferson and James Madison in 1796. The most recent Democratic Party president was Bill Clinton who ruled from 1992-2000.
Many American voters retain a loyalty for a particular political party regardless of the candidate. Therefore, the candidate who can appeal to traditional supporters of the opposing party will untimately be successful. The republicans are the imcumbent party and history tells us this may be a disadvantage. The incumbent parties votes usually drop as the electorate are looking for fresh faces and fresh ideas. Time will tell.

Democratic Party- www.democrats.org/
Republican Party- www.republicans.org/