Sen. Barack Obama of Illinois on Friday urged hundreds of blacks not to vote along racial lines next week in Maryland's Senate race.

Obama, the only black U.S. senator, came to the state to rally support for Democratic Rep. Ben Cardin, who is white. Cardin's Republican opponent, Lt. Gov. Michael Steele, is the first black candidate ever elected statewide and has been courting black Democrats.

"Listen, I think it's great that the Republican Party has discovered black people," Obama said to laughter from students at the rally at predominantly black Bowie State University. "But here's the thing. ... You don't vote for somebody because of what they look like. You vote for somebody because of what they stand for."

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Steele and Cardin are competing for the seat left open by retiring Democrat Paul Sarbanes in predominantly Democratic Maryland. Democrats must gain six seats to take control of the Senate.

A Baltimore Sun poll this week showed Cardin leading Steele by 6 points, but the race was tightening. The poll found Cardin had strong support among black voters, while Steele had strong support among white voters and rural residents. The poll had a margin of error of plus or minus 3.5 points.

In the campaign's closing days, both Steele and C ardin have focused on attracting black voters. Democrats are hoping for strong turnout in a traditional base of support, while Steele believes success in the state requires peeling away black voters who don't usually vote for the GOP.

Steele has campaigned heavily in his home county of Prince George's, which has more than 800,000 residents and is almost two-thirds black.

At a debate Friday, Cardin criticized a Republican handbook for poll watchers as a GOP attempt to suppress voter turnout and challenged Steele to repudiate the call by party officials to challenge the credentials of voters.

Steele did not respond to Cardin's statement during the debate, but later told reporters that he had not seen the handbook and that the state party needs to address any concerns that voters may have about it.

The instructions distributed by the state GOP advise poll watchers that their most important duty "is to challenge people who present themselves to vote but who are not authorized to vote." Cardin called it "voter suppression."

State GOP officials have said the poll watchers' duty will be to ensure that people are not allowed to vote if they are not registered, not to challenge legally qualified voters.

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