Missouri Attorney General Chris Koster praised U.S. Sen. Claire McCaskill on Monday as the “heart and soul” of the state’s Democratic Party.

The statement came after the second-term U.S. Senator announced that she would not challenge him as he prepares to seek the party’s nomination for governor in 2016.

“Claire McCaskill is the heart and soul of the Democratic Party in Missouri, and her commitment to progress for our state is unsurpassed. Creating a better future for Missouri is at the center of everything she does,” Koster said in a written statement.

“Senator McCaskill is a friend and advisor to me, and it is a friendship I hope continues for many years to come. Claire’s voice for centrist principles will move the U.S. Senate and our country forward,” he added.