WASHINGTON (AP) — President Barack Obama has announced he is running for re-election and is setting into motion a vast campaign machine that some expect to raise an unprecedented $1 billion.

Such a campaign war chest will mount a daunting challenge for whoever emerges from a crowded Republican field still jostling for the nomination and the difficult task that goes with it of removing a White House incumbent.

He raised $750 million for his 2008 campaign.

Obama launched his run at a second term in an email sent early April 4 to 13 million supporters. He exhorted them to join forces behind his leadership and the changes he has battled for in a nation riven with hyper-partisan political divisions.

The president formally stepped onto the long road toward the November 2012 election even as he was enmeshed in a monumental battle over how much and on what the federal government spends taxpayer dollars.

Obama said he was running again because 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.”

Despite acute anxiety among Americans about the economy, polling shows Obama’s standing is holding fairly high. The latest Associated Press-GFK survey showed half of those questioned still approve of the job the president is doing and half say he deserves to be re-elected. His rating on handling the economy stood at 47 percent approval. Twice as many people said Obama “understands the important issues the country will need to focus on during the next two years” as said that about Republicans in Congress.

Among the tangle of Republican opponents positioning themselves to challenge Obama in presidential balloting that still is 19 months away is former Minnesota Gov. Tim Pawlenty. He 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 Speaker Newt Gingrich were expected to join the race. Media phenomenon, former 2008 vice presidential candidate and tea party favorite 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, announced 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 his next moves in the long Afghan war and the removal of the last American forces from the conflict in Iraq.

Between now and November 2012, 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. Republicans, carried along on the heat from their new tea party allies in Congress, have made it their mission to kill the program

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.

The announcement also coincided 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.