Tax returns became an issue Wednesday in the confirmation hearing of Rex Tillerson, the former oil company executive who is President-elect Donald Trump's nominee for secretary of state – and the Senate committee questioning him wasn't just interested in his own.

Several Democratic senators on the committee asked Tillerson, who was CEO of Exxon Mobil Corp., if he would give them his tax returns, pointing out that neither he nor Trump have handed over information that could reveal potential conflicts of interest among their financial holdings.

Sen. TIm Kaine of Virginia, the former running mate of Democratic presidential nominee Hillary Clinton, aggressively questioned Tillerson about the issue – and whether Trump's steadfast refusal to release his tax returns would create "pressure points" that could undermine or cripple Tillerson's diplomatic positions with other nations, should he be confirmed.

"I have no direct knowledge" that it would hamstring influence overseas, Tillerson said.

During the second round of questions, Sen. Ben Cardin, a Maryland Democrat and the ranking minority member, asked Tillerson about his finances, noting that the nominee still has real estate holdings and operates a charity as well as a private farm.

"We are concerned ... that you will have some private interests worth millions, Cardin said. "We need a better understanding of how that operates."

Sen. Bob Corker, a Tennessee Republican and the committee chairman, jumped in, warning Cardin of asking "gotcha" questions of the nominee. Corker noted that the committee had not asked Tillerson to submit his taxes.

The issue surfaced a third time when Sen. Jeanne Shaheen, a New Hampshire Democrat, wanted to know how Tillerson felt about financial conflicts of interest and whether he could set aside his portfolio, avoid conflicts of interest and put the interests of his country first.