The board of the N.A.A.C.P. voted to endorse marriage equality on Saturday, putting the weight of the country’s most prominent civil rights group behind the cause. The largely symbolic move, made at the group’s quarterly board meeting in Miami, puts the N.A.A.C.P. in line with President Obama, who endorsed marriage a little over a week ago. Given the timing, it is likely to be viewed as both a statement of principle as well as support for the president’s position in the middle of a closely contested presidential campaign.