CHARLOTTE, N.C. (AP) — His re-election in doubt, President Barack Obama aimed to persuade economically strapped middle class America to return him to the White House in a prime-time speech Thursday night closing out the Democratic National Convention.

"Nothing's more powerful than voices calling for change," Obama told supporters on a pre-speech conference call, reasserting ownership of his optimistic slogan from the 2008 campaign after four tumultuous years in office.

The convention's final night also included an acceptance speech from Vice President Joe Biden.