Senator Barak Obama (left) with John Edwards, 14 Apr 2008

Former U.S. presidential hopeful John Edwards has endorsed Barack Obama for the Democratic presidential nomination, saying it is time for the party to come together behind the Illinois Senator.

Edwards, a former senator from North Carolina, told a cheering crowd in a stadium in Grand Rapids, Michigan Wednesday that Obama can create lasting change in America by "taking down the walls" that divide Americans along class lines.

Edwards praised Obama's rival, Senator Hillary Clinton, but said it was time for Democrats to unite behind one candidate in order to defeat the presumptive Republican nominee John McCain.

Earlier Wednesday, Clinton vowed to press on with her campaign, following a landslide victory in Tuesday's primary in the eastern state of West Virginia.

But the win does little to help Clinton catch up Obama, in the total number of delegates needed to secure the nomination.

Obama continues to widen his superdelegate lead on Clinton, picking up the support of three more, Congressman Peter Visclosky, Democrats Abroad chair Christine Schon Marques and College Democrats vice president Awais Khaleel Wednesday.

Superdelegates are party officials and elected office holders who are free to vote for either candidate at the party's nominating convention in August.

Separately, Obama also won the endorsement Wednesday of a leading abortion rights group, NARAL pro-choice America. The group had previously supported Clinton.

Looking ahead to the November general election, a new poll out of Quinnipiac University shows voters favor either Democratic candidate over presumptive Republican nominee John McCain.