WASHINGTON — US President Barack Obama admitted he had lost some of his youthful zest, but called on his grass-roots army to lift him to a second term in the White House.

Obama also said on a conference call that his "day job" battling global crises from the slow economic recovery in Washington to unrest rocking the Middle East would limit his time on the 2012 campaign trail at first.

But taking time out from a budget row with Republicans, he urged supporters to act as his "mini-campaign managers" outside the Washington bubble to keep his hopes on track.

"Look, over the last two years, I have aged a little bit, I've got gray hair, got bags under my eyes now, you know, some of that youthful appeal may be faded a little bit," 49-year-old Obama said.

"But something that I have never lost is my belief in you."

Obama's two years in the White House have seen one of the most turbulent baptisms of fire for any new president in modern memory.

The massive expectations that greeted Obama's election as America's first black president are now a memory, and he must forge his reelection bid while continuing the grim task of governing through a tough time for his country.

The purpose of Tuesday's call was to urge supporters to rebuild the massive grass roots political organization that rewrote political logic in 2008 and turned Obama from a freshman senator into America's 44th president.

Obama's Chicago-based campaign wants to rebuild its huge network in small towns and cities across the country, and to begin the task of raising money for what could be a billion-dollar war chest.

"What made our campaign different, was it wasn't about me it was about you," Obama said, seeking to conjure up the spirit of community and spirit that underpinned his first White House race.

Vice President Joe Biden was also on the call.

"My message to you is very, very simple. You need to do it again in order for us to be able to win again," Biden said.

Obama on Monday became the first candidate to formally enter the race to the November 2012 presidential election but the Republican race is beginning to simmer ahead of a primary campaign to chose the party's nominee.

Former Minnesota governor Tim Pawlenty has been campaigning hard while former Massachusetts governor Mitt Romney is on the road and contemplating a second run for the nomination after his failed bid in 2008.

The crop of possible Republican candidates also includes former House speaker Newt Gingrich, Mississippi Governor Haley Barbour, folksy Mike Huckabee and outgoing US ambassador to China Jon Huntsman.

Conservative favorite and 2008 vice presidential nominee Sarah Palin has yet to tip her hand, and a new female star is emerging: Tea Party favorite Michele Bachmann.