Hillary Clinton and Barack Obama have shifted their battle for the Democratic presidential nomination to Indiana, the mid-western state where the former first lady faces yet another must-win vote in the opening days of May.

Coming out of a solid victory this week in Pennsylvania, Clinton faces an uphill contest in Indiana where polls show Obama holds a slight edge.

The Illinois senator also holds a comfortable edge in North Carolina, a southern state with a large African American population, which votes in tandem with Indiana on May 6.

While Clinton's victory in Pennsylvania breathed new life and fresh cash into her campaign, she only dented Obama's nearly unassailable lead in elected delegates.

And as she moved west into Indiana, Clinton ignored the long odds and declared herself the best candidate to defeat Arizona Senator John McCain, who wrapped up the Republican nomination about two months ago and has been benefiting greatly from the increasingly bitter battle between the Democrats.

In Indianapolis, Clinton pledged to focus on economic issues.

"This campaign for me here in Indiana is about jobs, jobs, jobs and jobs," the two-term New York senator said, promising to invest in manufacturing and end tax breaks for companies that ship jobs overseas.

In New Albany, Indiana, Obama was asked why he thought he could win Indiana after losing Pennsylvania and Ohio, two states with large numbers of working-class voters.

"People are a little more familiar with me here," he said.

Indiana flanks his home state of Illinois on its eastern border and shares a northern television market next to the Chicago metropolitan region.

Clinton is running a funding deficit against the cash-flush Obama, and as soon as her victory in Pennsylvania was known, she launched an internet fundraising campaign.

Her campaign said that Clinton raised $US10 million ($A10.55 million) in the 24 hours after winning the primary, with contributions from 80,000 new donors out of 100,000.

Clinton's popular vote margin in Pennsylvania of more than nine percentage points, gave her at least 82 of the 158 delegates, according to an analysis of election returns by The Associated Press.

Obama won at least 73, with three still to be awarded.

In the overall race for the nomination, Obama leads with 1,723.5 delegates, including superdelegates. Clinton had 1,593.5, according to the AP tally.

McCain, meanwhile, sought to strengthen his credentials as an unconventional Republican, campaigning in a poor region of Kentucky following stops earlier in the week in Selma, Alabama, site of a historic civil rights march, and Youngstown, Ohio, a struggling industrial city.

He spent part of Wednesday in a dispute, unsuccessfully urging the North Carolina Republican Party not to air a commercial that shows Obama's former minister, Reverend Jeremiah Wright, denouncing the United States from the pulpit.

The commercial says both Democratic candidates in the state gubernatorial primary support Obama, whom it labels "just too extreme for North Carolina."

"The television advertisement you are planning to air degrades our civics and distracts us from the very real differences we have with the Democrats," McCain wrote to Linda Daves, North Carolina party chairwoman.

"In the strongest terms, I implore you to not run this advertisement."

Daves turned the request aside, saying, "It is entirely appropriate for voters to evaluate candidates based on their past associations."