"No success in my life has ever come without a good fight," he said, announcing his bid.
"I'm ready. I am more than ready," he said.
Activists are hitting the Democratic Senate hopeful over his record on entitlements.
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.