Obama's announcement, made through an email and video sent to supporters, set in motion a plan to tap donors and raise as much as US$1 billion, which would shatter the US$750 million campaign finance record he set in 2008.

He urged supporters in a phone call on April 4 evening to work to preserve his administration's policy initiatives, and he promised to work on issues such as energy and immigration reform over the next year.

"We can't go backwards," he said. "We have to preserve the progress that we've made and take it to the next level, and that means that we're going to have to mobilize."

Obama became the first black US president in 2009, and scored big legislative victories when Congress approved reforms of healthcare and financial regulation laws last year. But the economy has been slow to recover from recession despite a stimulus package of more than US$800 billion.

Obama's path to re-election will depend greatly on how he fares with independent voters, who were crucial to his 2008 victory but who abandoned Democrats last November.

The president has adopted a more centrist tone in recent months in response to that midterm election loss, emphasising his desire to work with both Democrats and Republicans.

Five months after his Democrats were routed by Republicans in November congressional elections, Obama looks in fairly good shape for re-election when paired against any of a group of potential Republican challengers.