Attorney General Martha Coakley announced Thursday she will run as a Democratic candidate in the special election to succeed the late Sen. Edward M. Kennedy.
The 20-year prosecutor said she can continue to be "an effective voice for the people of Massachusetts."
Kennedy died last week of brain cancer at age 77.
"As some have noted, we perhaps cannot fill his shoes, but must strive to follow in his footsteps," Coakley said in remarks prepared to deliver to supporters at a downtown Boston hotel.
The 56-year-old Coakley becomes the most prominent candidate to officially declare. Several others are waiting for Kennedy's nephew, former Rep. Joseph P. Kennedy II, to decide if he will run.
Coakley said she decided to run "because government should work well and work for everyone," adding that the performance of government "seems at times disheartening and discouraging."
"I believe now is the time to move beyond the idea of `good enough' government and demand a new standard of excellence. I know that I need to prove to voters that I am the best candidate in this race and I believe I can do that," Coakley said.
Coakley described her humble roots in Western Massachusetts and her career as a prosecutor, both as Middlesex District Attorney in a large district that includes populous suburbs of Boston, and since, 2007, as the state's attorney general.
"Now I hope to bring my experience to Washington," she said.
She did not overtly highlight her status as the state's top female elected official.