Voters in seven states and Washington D.C. are heading to the polls. It is the last big primary day before November's midterm elections.
Those states include Democrat-dominated Delaware.
GOP party leaders think eight-term Congressman Mike Castle is their only hope of winning Vice President Biden's old Senate seat, but he is in a tight race with Christine O'Donnell. She has lost Senate bids twice before, but this time has a Tea Party backing and support from one heavy hitter.
A Tea Party favorite is also making for a tight race in New Hampshire's Senate race.
Tea Party candidates have already won Senate primaries in six states this year.