Senator Hillary Rodham Clinton has lent her presidential campaign $US6.4 million ($A6.74 million) over the past month, her campaign says, underscoring the financial advantage held by her rival, Barack Obama.

The money more than doubled Clinton's personal investment in her bid for the Democratic nomination. She gave her campaign $US5 million earlier this year.

A campaign aide said Clinton gave her campaign another $US5 million on April 11, more than a week before the Pennsylvania primary. She then again dipped into her personal wealth for $US1 million last week and $US425,000 on Monday, one day before the North Carolina and Indiana primaries.

Clinton's campaign reported raising $US10 million online after her Pennsylvania victory on April 22. Evidently, the money was not enough and her fundraising was unable to keep up with her expenses heading into Tuesday's contests.

Moreover, Obama has routinely outspent her in primary after primary and has shown little difficulty tapping his vast network of donors. He spent more than $US7 million on advertising ahead of Tuesday's primaries in North Carolina and Indiana to her nearly $US4 million.

According to the latest campaign finance reports filed with the Federal Election Commission, Obama began the month of April with $US42 million in the bank for the primary to Clinton's $US9.3 million.

But Clinton had debts of $US10.3 million at the start of the month, much of it money owed to her main polling, phone banking and advertising consultants.

Hillary Clinton battled for a victory in Indiana that would keep her faint White House hopes alive.

With a double-digit win over Clinton in North Carolina's nominating contest, Obama rebounded from a rough campaign stretch fuelled by his comments on "bitter" small-town residents and a controversy over racially charged comments by his former pastor, the Reverend Jeremiah Wright.

In Indiana, Clinton led Obama by about 23,000 votes out of more than 1.2 million votes cast with nearly all of the ballots counted early Wednesday.

The two Democrats are embroiled in a gruelling battle for the right to represent the party in November's presidential election against Republican John McCain.