WASHINGTON - Hillary Rodham Clinton's decision to lend her presidential campaign $6.4 million from assets she holds jointly with her husband is rekindling questions about millions of dollars that special interests have paid Bill Clinton for speeches and other work since he left the White House.

In tapping some of that cash, "the Clintons have effectively bypassed campaign finance reform in a manner that's ingenious -- using Bill Clinton effectively as a front for the fundraising," said Lawrence Jacobs, a University of Minnesota political science professor.

Beginning days after he left the White House in 2001, the former president has been circling the globe, speaking roughly 250 times on tours that brought him more than $40 million in six years.

The sponsors have included investment banks that later suffered billions of dollars in losses in the subprime mortgage debacle and now have a big stake in any regulatory changes; an insurance group with an interest in any overhaul of the nation's health-care system; a group that favors the reunification of Taiwan with mainland China; a Colombian business-development group that backs a free-trade agreement; and more than two dozen Jewish groups, synagogues and museums.

Clinton campaign spokesman Jay Carson dismissed such concerns, saying, "There are no conflicts of interest, and every dollar either of them have made is all publicly available."

However, said Jacobs, the director of the University of Minnesota's Center for the Study of Politics and Governance, "There appear to be a number of prominent, wealthy corporations in the financial services sector, the health-care sector and others that stand to gain considerably from the election of Hillary Clinton as president. If all of these groups were giving to her directly, there would be all sorts of questions raised."

Questions about family income also have dogged the presumptive Republican presidential nominee, Arizona Sen. John McCain. The Democratic National Committee on Thursday prodded McCain to release the tax returns of his wealthy wife, Cindy.

On NBC's "Today Show" Thursday morning, Cindy McCain said that she would never release her tax returns.

When Hillary Clinton lent her campaign $5 million in early February, she made it clear that those funds came from her own money.

But her last-ditch decision to tap joint funds to remain financially competitive with rival Barack Obama exposes her husband's financial dealings over the last seven years to greater scrutiny.

On Wednesday, when Clinton spokesman Howard Wolfson disclosed that she'd lent her campaign $6,425,000 more since April 11, he said that the funds came from their shared assets.

"Legally, she is entitled to use up to 50 percent of their jointly held assets for her campaign, if she chooses," Wolfson said. "Those are the rules."