(CNN) - Democratic Sen. Michael Bennet of Colorado already has half a dozen Republican candidates who say they'll challenge him next year when he's up for election.
Now, he has a challenger from his own party as well.
Former Colorado House Speaker Andrew Romanoff is expected to officially announce his bid for the Democratic Senate nomination, with a formal announcement scheduled for Pueblo this morning, followed by events later in the day in Colorado Springs and Denver.
Bennet was considered by many to be a surprise choice by Colorado Gov. Bill Ritter to replace Sen. Ken Salazar, who stepped down from his seat in January to serve as President Barack Obama's Interior Secretary. Bennet, who was superintendent of the Denver public school system, will have to defend the seat next year.