A social activist and perennial candidate who is currently running for the U.S. Senate was stabbed multiple times in an attack in his home, authorities said.

Robert Kaufman (search), 74, was in serious but stable condition Tuesday at the University of Maryland Hospital, where he was rushed after the attack Monday afternoon.

Investigators believe the stabbing stemmed from a landlord-tenant dispute.

"We don't have any suspects in custody, but we do have good leads," police spokesman Troy Harris said. Kaufman rents out several rooms in his home and owns a few nearby buildings with tenants, said Michael Melick, a member of the City Wide Coalition, Kaufman's activist organization.

Kaufman is one of three declared candidates in the Democratic primary for the seat being vacated by Sen. Paul Sarbanes (search), who is retiring after next year's election. Rep. Ben Cardin (search) and former NAACP head Kweisi Mfume (search) also are running.

Kaufman got 7 percent of the vote when he ran in 2004 in the Democratic primary against Sen. Barbara Mikulski (search). He described himself as the "bring the troops home from Iraq now, end the phony war on drugs, and end the suicidal war on terrorism now candidate."

He also has run for Baltimore mayor and for Congress.