Michelle fires up Democrats

US First Lady Michelle Obama has made an impassioned speech backing her husband, President Barack Obama, for another four-year White House term.

Closing the first night of the Democratic convention, Obama spoke of the couple's shared background in struggling families.

Obama said four years ago she believed deeply in her husband's vision for the country but worried about how a run for president would change their life and that of their daughters.

President Obama will formally accept the nomination on Thursday, and face Republican Mitt Romney in November.