Rep. Bruce Braley, D-Iowa, told supporters Thursday he is running for Senate.

The four-term congressman’s entrance into the race gives Democrats a top-tier candidate to hold the seat of retiring Sen. Tom Harkin, D-Iowa, who announced last month he will not seek a sixth term.

“It’s a big responsibility to represent the people of Iowa in the United States Senate, especially after Tom Harkin has shown us how for the last 30 years,” Braley said in the email. “But, if you are willing to help me, I’m ready to go.”

Braley, who had previously acknowledged his interest in the race, starts as the heavy favorite for the Democratic nomination. It’s the state’s first open Senate seat in four decades, and Roll Call rates it as Leans Democratic.

There are several interested Republicans looking at the race.

A poll released earlier this month found GOP Reps. Steve King and Tom Latham would have a competitive nominating fight if both decided to run.