"I have simply become fed up with what I feel is a pervasive level of corruption in Pennsylvania," he said in an interview Tuesday. "The Attorney General's Office has been a do-nothing office for too long."
Bailey's candidacy complicates the two-way race that Democratic Party officials hoped would narrow as it advanced toward the April 24 primary.
Last month, the party failed to endorse any of the three candidates seeking its nomination at the time - Murphy, Kane, and former Philadelphia prosecutor Dan McCaffery.
McCaffery has since dropped out, and Kane and Murphy have turned their attacks on each other.
They responded to Bailey's candidacy Tuesday with first a yawn and then a swipe at each other.
"It doesn't change the fact that this is really a two-person race between Patrick, who has dedicated his life to protecting middle-class families, and Kane, who is trying to buy this election using money from her antiunion company," Murphy campaign spokesman Nat Binns said.
Kane's camp, meanwhile, launched a familiar attack at the new target.
"Regardless of how many congressmen seek this office, we believe Pennsylvania Democrats want to nominate a career prosecutor, not a career politician," spokesman Josh Morrow said, repeating a line Kane has repeatedly used to attack Murphy's credentials.
Bailey acknowledged his late entrance could relegate his candidacy to long-shot status. His opponents already have campaign war chests worth millions.
But he hopes his name recognition will help give him a boost in the polls.
Bailey represented Westmoreland County in Congress for two terms in the late '70s and early '80s before becoming a victim to redistricting and a 1982 primary loss to fellow incumbent Rep. John Murtha.
He was elected to the Auditor General's Office in 1984, and lost a primary bid for U.S. senator two years later. Since losing his reelection bid, he has launched unsuccessful primary campaigns for auditor general in 1992 and governor in 1998.
His Harrisburg-based civil-rights law practice has occupied most of his time during the last decade, he said. Focused on representing whistle-blowers, Bailey describes himself as an "equal-opportunity suer."
However, that penchant to file first and ask questions later nearly resulted in his disbarment last year, after he accused seven federal judges of conspiring against him in a lawsuit.
For his part, Bailey maintains that the handling of that case and the treatment of his clients in others represents a corruption of the state and federal judicial system.
"It's politics as usual in Pennsylvania," he said. "I'm not going to accept the same old thing."
Contact Jeremy Roebuck at 267-564-5218 or email@example.com, or follow on Twitter @jeremyrroebuck.