Since 1980, there have been just eight serious primary campaigns mounted against incumbent U.S. senators (five of which succeeded and three of which didn’t). But next year alone, we could be looking at as many as six. The incumbents most vulnerable to serious intraparty competition in 2010 include four Democrats—Kirsten Gillibrand in New York, Chris Dodd in Connecticut, Arlen Specter in Pennsylvania and Roland Burris in Illinois—and two Republicans, Kentucky’s Jim Bunning and Arizona’s John McCain. Granted, they don’t all face the same degree of threat. McCain, for instance, has already drawn two challengers, each eager to exploit the independent streak that has made him an enemy to some on the right.