Liu, 44, is a former Bush Justice Department deputy assistant attorney general who served on Trump's Justice Department transition team and as deputy general counsel at the Treasury Department.
Feinstein, who spoke on behalf of Democrats about Liu, called that meeting uncommon and said "a pre-nomination between a sitting president and a U.S. attorney nominee is unusual regardless of the district that the U.S. attorney candidate might represent. And as I said, we've checked, and to our best knowledge this has never happened in the last administration."
Liu's April 21 meeting at the White House was disclosed after committee Democrats sent questions to the administration's first seven U.S. attorney nominees in June. Liu was the only one who answered that she "interviewed with representatives of the White House Counsel's Office and then met the president with the White House counsel," Donald McGahn.
Committee Democrats acted after Trump said he fired FBI Director James B. Comey because of his handling of an investigation into Trump campaign dealings with Russia, accused special counsel Robert S. Mueller III of conducting a "witch hunt" in the probe, called his attorney general "beleaguered" and "weak," and criticized federal judges since taking office in January.
The confrontations have raised concerns among critics in both parties that White House actions undermine the rule of law by blurring the line between the president and an independent Justice Department.
Liu was nominated by the White House on June 12 to succeed Channing D. Phillips to lead a U.S. attorney's office whose 300 attorneys have jurisdiction to prosecute local crimes in the nation's capital along with federal crimes.
Liu's confirmation vote before the Senate has not been set but is expected after Congress returns in September. She has declined to comment while her nomination is pending.
Liu, a graduate of Yale Law School, was an assistant U.S. attorney in the District from 2002 to 2006, before joining the Justice Department's national security division and serving as a deputy assistant attorney general with the civil rights division until 2009.
She also worked at the Morrison & Foerster and Jenner & Block law firms.
"Given the unique role that the U.S. attorney for the District of Columbia may have in investigations that directly related to the administration, I just want to point out that it's concerning that she [Liu] was singled out for a meeting with the president," Feinstein said in opening remarks.
A spokesman for the White House did not immediately respond to a request for comment.
Feinstein said she voted for Liu because in response to the committee's questions, Liu made clear that in the April 21 meeting, no one raised the subject of any current or potential investigations of the president, his family members, Cabinet members or businesses.
Feinstein said Liu also committed to following Justice Department policies regarding communications with the White House.
Candidates for the 93 U.S. attorney positions typically are interviewed by Justice Department officials such as the attorney general and White House counsel staff, more than a half-dozen former federal prosecutors and White House officials said. Meetings with the president and counsel are typically reserved for top administration and judicial nominees, they said.
Administration nominees to the U.S. attorney post in the District draw added scrutiny because unlike the 50 states, the District has no senators — who could threaten to block confirmation — with whom the administration typically consults on its nominee.
U.S. attorneys who have held the job in the past have gone on to serve in senior administration positions, including Obama administration attorney general Eric H. Holder Jr., and George W. Bush homeland security adviser Kenneth L. Wainstein.
Several of Liu's former mentors and superiors appointed by presidents of both parties praised her selection as an excellent lawyer, hard-working manager and not visibly partisan.
In statements, Del. Eleanor Holmes Norton (D-D.C.) has criticized the Trump administration for breaking with recent predecessors in not consulting with her before making the nomination and not honoring the principle of local residency.
Liu lives in Arlington.