Massachusetts voters head to the polls Tuesday to fill the first open US Senate seat in 25 years in a race that started out slow but has become the hottest item in national politics.

Some polls show Republican candidate Scott Brown and Democratic hopeful Martha Coakley running neck-and-neck in their campaigns for Edward M. Kennedy's seat. Additionally, the outcome of the election could reverse or preserve the filibuster-proof majority the Democrats currently enjoy in the Senate.

So, what's going to decide this election? Here are six factors to watch on Tuesday.