WASHINGTON — On a day that saw President Donald Trump's nominee to head the Central Intelligence Agency gain crucial Senate support, one critic demanded to know whether the nation's spy agency conducted surveillance on the president during the 2016 election.

In a letter to CIA director-nominee Gina Haspel, Sen. Rand Paul, R-Ky., asked for information on the agency's activities during the 2016 presidential campaign, specifically whether Trump was being monitored during overseas travel.

“It is supposed to be illegal for the CIA to spy on Americans – so the question is did John Brennan ask the British intelligence to spy on Americans for him? Did he ask them to spy on President Trump’s campaign?” Paul said of the former CIA Director in an interview with NBC News.

The letter asks for “clarification” on CIA practices on monitoring presidential candidates and what Hapsel knew about the topic. It was sent on the same day that Haspel publicly called the agency's enhanced interrogation methods of the past a mistake, leading to a number of Democrats announcing they would support her now-likely confirmation.

Paul has been an outspoken critic of Haspel and has opposed her nomination since it was announced.

Paul said the president, who nominated Haspel, has expressed his concern over the issue.

“The president is very concerned about this,” Paul said. “And I think my concern is Gina Haspel has been in the very highest ranking in the CIA, Did she know anything about it? So I sent a letter to Gina Haspel asking her did the CIA eavesdrop on the Trump campaign when they were in England?”

Trump has in the past accused President Barack Obama of wiretapping Trump Tower — a claim that has been refuted by intelligence officials but one he's asked Congress to investigate.

A senior administration official reiterated the president “is fully behind Haspel.”

In an interview with NBC News, Paul referenced a New Yorker story which reported that Robert Hannigan, the head of the British intelligence service, flew to Washington, D.C. to brief then-CIA director John Brennan on “a stream of illicit communications between Trump’s team and Moscow that had been intercepted.”

The letter asks Haspel: “Have you, or anyone else at the CIA, ever cooperated with any foreign intelligence services to surveil, monitor, or collect any information on candidate Trump during his travels outside the United States in the preceding five years? Specifically, was candidate Trump ever under any surveillance or of interest to the CIA during his previous visits to Europe?”

The Senate Intelligence Committee is slated to hold a vote on Haspel’s nomination Wednesday morning. The top Democrat on the committee, Sen. Mark Warner, D-Va., announced on Tuesday that he’d support her nomination. His support came after she wrote him a letter saying that the CIA’s use of harsh interrogations in the years after the September 11th attacks were a “mistake.” It was her most direct public repudiation of the program since her nomination.

Warner’s announcement of support was quickly followed by more Democratic senators doing so, including Sens. Heidi Heitkamp, D-N.D, and Bill Nelson, D-Fla.