Della, 48, was a member of the Baltimore City Council from 1976 to 1983 and a member of the state Senate since 1983.
"I just think there's a void that needs to be filled, and I believe I can fill that void," Della said today, explaining his decision to challenge Clarke.
Della recently said Clarke was "probably the hardest-working person I've ever seen in the office." But he has been critical of her successful push to win council approval of a controversial redistricting plan.
"I believe some of the things that recently transpired are outrageous," Della said today, adding that as council president he would serve as a liaison between the city and the governor and key legislators.
Della filed his official candidacy papers today, the last day he could file to run in the Sept. 12 Democratic primary.
Della will apparently be the only Democratic challenger for Clarke, who is seeking a second term as council president and is considered one of the most popular politicians in Baltimore.
Some elected officials, including state Sen. American Joe Miedusiewski, D-City, have been searching for a candidate to challenge Clarke.