Southbound, Clinton aims to build delegate edge over Sanders

"Because we can do the long game, once we get past March 1, the calendar changes dramatically," said Jeff Weaver, Sanders' campaign manager.

More than half the 2,383 delegates needed to win the Democratic nomination will be determined in the 28 states that hold primaries and caucuses in March.

Clinton's campaign failed to account for the Democratic Party's system of allocating delegates proportionally in voting contests, then watched superdelegates, who can shift their allegiances, move toward Obama as the campaign stretched late into the spring.

[...] he has energized young people, working-class voters and liberals with his impassioned calls for breaking up big Wall Street banks and making tuition at public colleges and universities free.

Sanders' prolific online fundraising has given him staying power and he has pledged to take his campaign into the Democratic convention in July.

[...] the high margin of error in the polls makes it impossible to say with confidence whether either candidate held a lead among the group.

Clinton's support among black voters could pay dividends because of the way Democrats award high-performing congressional districts with a greater share of delegates.