The incumbent senator is headed for a rematch with former Democratic Rep. Joe Sestak.
The former governor has an early lead over the incumbent senator.
"Morning Joe" host says race for Senate is possible.
Sen. Michael Bennet, who is up for reelection in 2016, will donate the money to charity.
The Ohio Republican raised nearly $3 million in the first quarter.
The Maryland Dem is the early favorite to replace retiring Sen. Mikulski.
The Democratic establishment is lining up against P.G. Sittenfeld, Strickland's young primary challenger.
A Chicago newspaper says the Democrat will challenge GOP Sen. Mark Kirk.
Outgoing Dem leader throws support behind ex-Attorney General Catherine Cortez Masto.
Democratic lawmaker said she talked to Reid shortly after he announced he'd retire.