Senator Barack Obama beat Senator Hillary Rodham Clinton on Tuesday in Mississippi’s Democratic primary, the final contest before what promises to be a crucial showdown in Pennsylvania in six weeks.

This would be Mr. Obama’s second straight victory  he won the Wyoming caucuses over the weekend  enabling him to rebound from a trio of popular vote losses to Mrs. Clinton in Texas, Ohio and Rhode Island.

Mr. Obama’s lead in Mississippi was built on a wave of support among blacks, who made up half those who turned out to vote Tuesday, according to surveys conducted by the television networks and The Associated Press of voters leaving polling places. The surveys found that roughly 90 percent of black voters supported Mr. Obama, but only a third of white voters supported him, suggesting a racially polarized electorate in the state.

“It’s just another win in our column and we are getting more delegates,” Mr. Obama said in an interview on CNN from Chicago, where he arrived Tuesday evening after spending the day campaigning in Mississippi and Pennsylvania. “I am grateful to the people of Mississippi for the wonderful support. Wat we’ve tried to do is steadily make sure that in each state we are making the case about the need for change in this country.”