U.S. Sen. Pat Toomey's rating from the National Rifle Association has dropped after the Pennsylvania Republican went against gun-rights groups and voted to support expanded background checks.

Toomey's new C-rating is a downgrade from the A-rating and hearty endorsement the NRA gave him when he last ran in 2010.

Toomey's Democratic challenger, Katie McGinty, drew an F-rating, unsurprising given her support for a much broader range of gun-control measures than Toomey supports.

The NRA didn't explain what considerations went into its rating.

The race is neck-and-neck and could help decide control of the U.S. Senate.

Toomey's vote to require background checks for online gun sales and purchases at gun shows earned him the endorsement of two prominent gun-control activists, billionaire Michael Bloomberg and former Democratic congresswoman Gabby Giffords.