"Having the endorsement of the Democrats' most recent governor provides the sort of imprimatur Mfume's political endorsements had to this point been lacking," says Tom Schaller, a political scientist at the University of Maryland-Baltimore County.

Glendening said in an interview that his endorsement was based largely on the expectation that Mfume, an African-American and former head of the NAACP, stands to do a better job appealing to Maryland's sizable black population than does Cardin, who is white. Such appeal will be particularly important in the general election, when the Democratic Senate nominee will face Republican Michael Steele, who is black.

"The overwhelming majority of the African-American support will go to the Democratic nominee, whether it's Mfume or Cardin," said Glendening, who served two terms as governor, from 1995 to 2003. "But if it's Mfume, the [black support] will return to its historical pattern, which is 90 to 95 percent support [for the Democratic nominee]."

Glendending said Democratic relations with the state's African-American community were bruised in 2002, when Democratic gubernatorial nominee Kathleen Kennedy Townsend selected a white Republican running mate. Her Republican opponent selected a black running mate, Steele, who is currently Maryland's lieutenant governor. Glendening said many blacks voted Republican in 2002, contributing to current Republican Gov. Robert Ehrlich's victory over Townsend.

"The reality is that no party can take any group for granted," Glendening said. "There was a tremendous decline in enthusiasm in the African-American community [after the 2002 election]."

As an outspoken environmentalist during his tenure as governor, and as an advocate for stepped-up environmental protections since then, Glendening said he hoped the endorsement would be especially effective among progressive undecided voters. Those voters are mostly white, and Mfume must expand his support beyond his African-American base if he hopes to win the September 12 primary. "My comments will focus on the environment and global warming," Glendening said of his appearance at this afternoon's press conference. "I'm talking about the progressive non-African-American community.

But the University of Maryland's Schaller said the Glendening endorsement was unlikely to help Mfume where he needs it most, in reaching white moderate Democrats. "The Glendening endorsement is somewhat redundant," Schaller said. "Mfume already has support from the more liberal elements in the Democratic coalition, such as the teachers association and [the liberal group] Progressives Maryland."