WASHINGTON (AP) — President Barack Obama formally launched his re-election campaign today. And he's urging grass-roots supporters central to his first White House run to mobilize again to protect the change he's brought over the past two years.

The official start of his second White House bid comes amid three wars, a budget fight with Congress, and sluggish economic recovery. It also comes 20 months before the November 2012 election.

Obama sent an e-mail to more than 13 million supporters announcing his candidacy. In it, he says lasting change never comes quickly or easily. And he says mobilizing needs to start to protect the progress that's already been made.

The campaign is based in Chicago. Many of the same people from his first bid remain involved, including former campaign manager David Plouffe, who now is in the White House, and chief political strategist David Axelrod.