Mfume cannot beat Steele statewide, period. There really isn't a whole lot else to say. If you know anything about politics and can bring your pro-Dem bias down from +8,000,000% for one second, it would be rather obvious.
Steele holds the entire GOP. Independents lean Steele. White Dems defect in relatively high quantities. Game over. He doesn't even need black votes.
Not so fast. There are a lot of liberal white Democrats and independents, especially in the DC suburbs (where I live), that will vote for Mfume if nominated. Whatever the race or personal characteristics of the candidates, these voters are not going to vote to give another Senate seat to the party of George W. Bush.
That said, I'm not sure if there are enough of those voters to put Mfume over the top. The Baltimore area has a lot of more conservative white Democrats and independents that will defect from Mfume, though not necessarily enough to swing the election. Most of them will likely stick by Cardin, however.