The decision likely clears the way for Attorney General Jack Conway to become the party's nominee for governor. The filing deadline is 4 p.m. Tuesday, and nearly every other potential Democratic challenger has announced plans not to run.
Grimes spent more than $18 million in 2014 trying to unseat the now-Senate Majority Leader Mitch McConnell in a race that garnered her national attention.
But a 15-point drubbing at the polls in November likely limited her options in 2015. Grimes does have an aggressive state legislative agenda planned that could set herself up for another attempt at federal office in 2016.