barack_obama_web_fc_4.jpgWASHINGTON (AP) _ President Barack Obama launched his re-election campaign Monday in an email to 13 million supporters, calling on them to join forces behind his leadership and the changes he has battled for in a nation riven with hyperpartisan political divisions.

Facing a monumental battle over how much and on what the federal government spends taxpayer dollars and a possible government shutdown for lack of funding, Obama said the fight was not finished in keeping the high-flown promises that carried him into the White House in 2008.

“We've always known that lasting change wouldn't come quickly or easily. It never does,'' Obama wrote in an email that he signed with his first name. “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.''

A tangle of potential Republican opponents are jockeying for position to challenge Obama in presidential balloting that still is 19 months away. Former Minnesota Gov. Tim Pawlenty, however, is the only mainstream candidate who has taken the needed first steps to open a campaign, filing papers that allow him to form what is known as an exploratory committee to raise money and hire staff.

About a dozen other Republicans, including former Massachusetts Gov. Mitt Romney and former House of Representatives Speak Newt Gingrich were expected to join the race. Media phenomenon and former 2008 vice presidential candidate Sarah Palin remains coy about a run for the White House.

Obama who has wrestled first to prevent a financial meltdown then to revive the badly damaged economy, is announcing his candidacy in the midst of a searing national debate over U.S. involvement in the Libyan uprising in North Africa. That military action is all the more complex as the president contemplates next moves in the long Afghan war and the removal of the last American forces from the conflict in Iraq.

Between now and the November 2012 election, the incumbent Democrat will work to convince a fickle America that he has delivered change, made the right moves and earned the chance to continue the job. He will have to defend policies that have proven divisive, chief among them his sweeping health care overhaul.

The announcement follows quick on the heals of good economic news on Friday when the government said the struggling economy showed more signs of a rebound with a report that the still high unemployment rate had fallen to 8.8 percent.

In the email, Obama directed backers to his new campaign website where a video featured supporters talking about their continued backing for the first African American president.

“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 liberals who have criticized some of his policies and independents who have abandoned him for more right wing, budget-cutting tea party messages.

Widely expected, the procedural step will allow the president to begin raising money in earnest for what allies say could be a record-breaking haul of more than $1 billion for his campaign. He plans visits to visit major money venues of Chicago, San Francisco and Los Angeles in the coming weeks.

The announcement also coincides with the date in 1968 on which American civil rights leader Martin Luther King was assassinated in Memphis, Tennessee.

The campaign is based in Chicago, and 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.

Managing the campaign this time is Jim Messina, who played a senior role in the first bid and in the White House. Messina has spent the past few months touring the country to lay the groundwork with donors in hopes of building a massive fundraising network featuring both large and small contributions. He has asked some 400 donors _ called bundlers _ to bring in at least $350,000 this year; the re-election website is geared toward raising money from backers. Obama raised $750 million for his 2008 campaign.

Obama faces no Democratic primary challenger.