Ed Markey at the Democratic National Convention. (Casey Cunningham/Washington Post)

Rep. Ed Markey (D) plans to run for the Massachusetts Senate seat likely being vacated by Sen. John Kerry (D), President Obama's nominee for secretary of state. 

Markey, 66, has served in the House since 1976 and is the dean of the Massachusetts delegation.  The veteran House member announced his decision in a statement, saying, “There is so much at stake.”