A federal watchdog agency responsible for reviewing the backgrounds of White House Cabinet nominees has warned that the office has been overwhelmed by the task of vetting Donald Trump’s selections.

In a letter to Democratic senators dated Saturday, the head of the Office of Government Ethics also warned that Republicans are trying to take the unprecedented step of holding hearings for Cabinet picks before they have completed requisite paperwork to ensure there are no ethical, financial or criminal concerns.

Walter M. Shaub Jr., the ethics director, said it is “of great concern to me” that several of Trump’s nominees have not completed an ethics review before hearings are scheduled to begin next week.

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Plans for at least seven Trump nominees to sit for hearings on Capitol Hill in the coming days have “created undue pressure on OGE’s staff and agency ethics officials to rush through these important reviews,” Shaub wrote. “More significantly, it has left some of the nominees with potentially unknown or unresolved ethics issues shortly before their scheduled hearings.”

Shaub added: “I am not aware of any occasion in the four decades since OGE was established when the Senate held a confirmation hearing before the nominee had completed the ethics review process.”

The OGE serves as the executive branch’s ethics clearinghouse both enforcing federal ethics rules and reviewing potential conflicts of interest for nominees to government posts. Shaub was appointed to lead the office in 2013. An attorney by trade, he donated a total of $500 to President Obama’s 2012 reelection campaign, federal elections records show.

Sen. Jeff Sessions (R-Ala.), Trump’s choice to be the next attorney general, and Rex Tillerson, the former ExxonMobil chief executive, whom Trump nominated to serve as secretary of state, are scheduled for confirmation hearings in the coming week. So is Betsy DeVos, a billionaire GOP power broker nominated to serve as education secretary.

Tillerson and DeVos are worth hundreds of millions if not billions of dollars, leading some Democrats to describe Trump’s nominees in recent days as “a bunch of billionaires.”

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The letter by Shaub was sent Saturday in response to queries by Senate Democrats. His concerns could undermine Republican hopes of swiftly holding hearings Tuesday, Wednesday and Thursday — around the same time that Trump is also expected to outline ways he will separate himself from his vast business holdings while serving as president.

The letter adds fuel to Democratic concerns that the incoming administration as well as congressional Republicans are attempting to rush the confirmation of Trump’s top picks.

The ethics office’s concern “makes crystal-clear that the transition team’s collusion with Senate Republicans to jam through these Cabinet nominees before they’ve been thoroughly vetted is unprecedented,” Senate Minority Leader Charles E. Schumer (D-N.Y.) said in response to the letter. “The Senate and the American people deserve to know that these Cabinet nominees have a plan to avoid any conflicts of interest, that they’re working on behalf of the American people and not their own bottom line, and that they plan to fully comply with the law. Senate Republicans should heed the advice of this independent office and stop trying to jam through unvetted nominees.”

It was not immediately clear what Senate Republicans might do. Aides to Majority Leader Mitch McConnell (R-Ky.) did not immediately return requests for comment.