Four years ago, Barack Obama captured 95 percent of the black vote. But in a 2012 election in which every vote may matter, Mitt Romney is not conceding an inevitable rout on that front.

On Wednesday, Mr. Romney will make a pitch to the nation’s premier civil rights group, the N.A.A.C.P., testing President Obama’s overwhelming support among black voters by trying to pry away some defectors with his pro-jobs message at a time of 14.4 percent unemployment among African-Americans.

Mr. Obama is passing up the chance to address the 103-year-old group at its annual convention in Houston and is sending Vice President Joseph R. Biden Jr. instead.

While blacks are expected to solidly back Mr. Obama again this year, he faces challenges in generating the same enthusiasm as in 2008. The level of black turnout could be especially crucial in states like North Carolina and Virginia, where black voters had an outsize influence in the president’s relatively narrow victories four years ago.