The nexus among private companies, Hillary Clinton’s State Department and the Clinton family foundations is closer and more complex than even Donald Trump has claimed so far.

While it is widely known that some companies and foreign governments gave money to the foundations, perhaps in an effort to gain favor, one of the key parts of the puzzle hasn’t been reported: At least a dozen of those same companies lobbied the State Department, using lobbyists who doubled as major Clinton campaign fundraisers.

Those companies gave as much as $16 million to the Clinton charities. At least four of the lobbyists they hired are “Hillblazers,” the Clinton campaign’s name for supporters who have raised $100,000 or more for her current White House race. Two of the four also raised funds for Clinton’s unsuccessful 2008 presidential bid.

USA TODAY reached these conclusions by obtaining federal lobbying data from the nonpartisan Center for Responsive Politics for 2009-2013, Clinton’s tenure as secretary of State. Reporters then compared the data with donor lists made public by the Clinton nonprofits and federal campaign financial records.

Some of the companies appear to have gotten what they wanted; others did not. The companies, which in several cases provided limited answers to detailed USA TODAY questions, said they had done nothing improper. The charity donations, though questioned by Clinton critics, were all legal.

"We have no record of Secretary Clinton meeting with these individuals as Secretary regarding issues they were lobbying on at the time. The fact remains, Hillary Clinton never took action as Secretary of State because of donations to the Clinton Foundation,” her campaign said.

Among the donors to the Clinton foundations who also used Clinton-connected lobbyists at the Department of State:

Microsoft has given between $1 million and $5 million to the foundations, as the tech giant also lobbied for visa issues, protection of critical infrastructure and cybersecurity, software industry licensing and government procurement.

Pfizer, one of the world’s top biopharmaceutical companies, has also given between $1 million and $5 million to the foundations, while lobbying for such issues as intellectual property rights overseas and issues related to medicines in Turkey and India.

ExxonMobil, the global oil and energy company based in Texas, gave the foundations between $1 million and $5 million. The company lobbied the Department of State for issues involving hydraulic fracturing, popularly known as fracking, oil sands and other provisions.

• The Northeast Maglev, a Washington, D.C.-based company that advocates for high-speed, magnetic levitation rail service in the U.S., donated as much as $100,000 while lobbying the Department of State to help provide support for the issue.

• Mexico TV network Azteca and its affiliates donated as much as $375,000 while lobbying for U.S. business opportunities, an education initiative involving students from the U.S., Mexico and Latin America, and other causes.

While the review did not find instances where companies received special favors, each example illustrates the unique challenge the Democratic presidential nominee would face in dealing with potential conflicts of interest if she were to win the White House.

If elected, Clinton would be the first U.S. president to have had previous involvement with a foundation that raised millions of dollars tied to foreign interests and other donors, said Douglas Brinkley, a history professor at Rice University.

To the extent that wealthy presidential candidates have been philanthropists, they typically relied on their own fortunes. For instance, H. Ross Perot, the billionaire businessman who ran as an independent in 1992, has self-funded his private foundation’s giving, disclosure records show.

In contrast, the Clinton nonprofits have been intertwined with the U.S. and global power structure. They have received millions of dollars in state or private contributions linked to Kuwait, the United Arab Emirates, Qatar, Kazakhstan and other nations whose interests at times conflict with those of the U.S.

Millions more have been donated by U.S. companies and special interest groups, many of which stood to benefit from decisions made by the Department of State.

“When you couple all of these activities together, it gives an unseemly appearance that this was another way for Clinton foundation donors to try to get what they wanted,” said Karen Hobert Flynn, president of Common Cause, a nonpartisan government watchdog. “I don’t see any quid pro quos. But I do think these are sophisticated lobbying operations by Clinton foundation donors trying to leverage Department of State support for whatever their pet projects are.”

Former president Bill Clinton announced in August that if his wife were elected, he would step down from the foundation’s board and no longer raise money for it. The foundation would also accept contributions only from U.S. citizens, permanent residents and U.S.-based independent foundations, while barring foreign and corporate donations, he said.

Responding to USA TODAY questions, Hillary Clinton’s campaign said she’s made no announcement about how she would deal with past donors to the foundation who might lobby her potential administration. Would these lobbyists be allowed to join her administration, a practice the Obama administration partly blocked? Would any restrictions be placed on past foundation donors?

Trump could face his own conflict of interest issues if the Republican presidential nominee were to win the White House. The businessman and reality TV star has business ties to companies both domestically and internationally, including Muslim nations in the Middle East. Trump has suggested that his children and business associates would run the Trump Organization if he were elected president.

“We’ve had wealthy presidents before, John Kennedy and FDR. But their wealth was much less, and was mostly domestic,” said Craig Holman of Public Citizen, a nonpartisan government watchdog. “Trump would be making foreign policy decisions that would be having an immediate impact on his personal wealth.”