Well this answers one question:
http://www.huffingtonpost.com/...
Sen. Barbara Mikulski's (D) decision to retire after five terms in office set off a wave of speculation about which of the myriad Maryland Democrats would run to replace her. On the top of that list was the state's former Gov. Martin O'Malley, who is eying another run for office and could, theoretically, decide that taking on Hillary Clinton for the presidential nomination is simply too tough a task.
Alas, on Tuesday morning, O’Malley released a statement fully ruling out a run in the 2016 Maryland Senate race.
“Senator Mikulski has done an outstanding job representing Maryland in the U.S. Senate for nearly 30 years," he said. "I am hopeful and confident that very capable public servants with a desire to serve in the Senate will step up as candidates for this important office. I will not be one of them.” - Huffington Post, 3/3/15
O'Malley would've been a terrific candidate but Democrats still have a deep bench. Plus O'Malley was eyeing a White House bid anyway. Stay tuned.