The top Republican senator overseeing the confirmation of ExxonMobil CEO Rex Tillerson to be secretary of state said Thursday that Tillerson will not be asked release his tax returns as part of his confirmation process.

Sen. Bob Corker's (R-TN) announcement comes after Democrats had been urging Corker to demand that that Tillerson's financials be thoroughly vetted.

Corker, head of the Senate Foreign Relations Committee, said in a statement that it has been "long-standing precedent" for the committee not to require nominees to release their taxes.

"He already has submitted a completed nominee questionnaire and will soon submit an extensive financial disclosure. Furthermore, prior to his confirmation hearing, he will go through the same ethics and FBI checks as previous Secretary of State nominees," Corker said.

"That has always been the plan, it is already in progress, and I am deeply disappointed my colleagues continue to imply otherwise," he added.

Sen. Ben Cardin, the top Democrat on the committee, and fellow Democrats have been demanding that Tillerson, the head of a a multinational corporation, release three years of his taxes as a prerequisite to be confirmed to the top diplomatic post.

Sean Bartlett, a spokesman to Cardin, said that a questionnaire section to be filled out by the nominee about taxes is "standard."

"The majority staff and the Trump transition team were informed by the minority staff that we were asking for (the tax returns) in person and in writing. We're still asking," Bartlett said.

While it is difficult for Democrats, who only have 48 members, to block Tillerson's confirmation, they could slow it down, especially if questions remain about Tillerson's personal financial interests.

A top senior Democratic aide said tax transparency is not an "unreasonable" request for this unique nominee.

"Given that he’s the CEO of a company that has investments in over 80 countries around the world, it’s not unreasonable to see relevant documents that will ensure he’s working on behalf of the country, and not his own bottom line," the aide wrote in an email. "It’s a shame he seems ready to take a play out of the President-elect’s playbook and withhold his taxes from the American people."

President-elect Donald Trump, also a businessman with financial interests around the world, did not release his taxes, breaking from protocol followed by nearly every presidential nominee since 1976 to do so.

Sen. Cardin wrote a letter to his colleagues Thursday saying he has made little headway with Corker over Tillerson's taxes.

Tillerson is required to fill out the standard, less detailed, financial disclosure form. Cardin wrote to his colleagues that he will wait to examine the financial disclosure documents before determining if he will object to the hearing date, which is expected to be scheduled for January 11.

"I think it is an important part of vetting this candidate because he has never made public disclosures of this type, as he has worked at ExxonMobil for his entire career and has never been in public service," Cardin wrote in the letter.

Republican leaders are hoping to have at least half a dozen of Trump's nominees confirmed before his inauguration on January 20.