Grass Roots by Stuart Woods

Grass Roots: A Will Lee Novel - Will Lee is committed by Judge Boggs to defending Larry Moody, a white man accused of raping and murdering a well known and liked black woman.  Will is currently Senator Carr's assistant and is looking to run for senate himself in a few years, however, Senator Carr becomes incapacitated and Will is catapulted into running for Carr's seat against Mack Dean.

Along with the senate race, there is also a religious supremacist group that is killing operators of businesses that don't meet their standards who is trying to get a man they can control into the senate.