President Barack Obama formally launched his re-election campaign Monday, 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, in the midst of three wars, a budget fight with Congress, and sluggish economic recovery, comes 20 months before the November 2012 election.

"We've always known that lasting change wouldn't come quickly or easily. It never does," the Democrat said in an e-mail announcing his candidacy to more than 13 million supporters. "But as my administration and folks across the country fight to protect the progress we've made — and make more — we also need to begin mobilizing for 2012, long before the time comes for me to begin campaigning in earnest."

He told them he was filing the necessary paperwork with the Federal Election Commission, and directed them to his new campaign website where a launch video featured clips from supporters talking about their continued backing of the Democrat.

"I don't agree with Obama on everything but I respect him and I trust him," Ed from North Carolina says, delivering what's certain to become a key part of the president's pitch as he tries to re-energize liberal backers who have criticized some of his policies and independent voters who have fled from him in his first term.