President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE on Thursday mourned the death of Rep. Elijah Cummings Elijah Eugene CummingsBlack GOP candidate accuses Behar of wearing black face in heated interview Overnight Health Care: US won't join global coronavirus vaccine initiative | Federal panel lays out initial priorities for COVID-19 vaccine distribution | NIH panel: 'Insufficient data' to show treatment touted by Trump works House Oversight Democrats to subpoena AbbVie in drug pricing probe MORE (D-Md.), tweeting that the late congressman and fierce critic of the administration would be "hard, if not impossible, to replace."

"My warmest condolences to the family and many friends of Congressman Elijah Cummings. I got to see first hand the strength, passion and wisdom of this highly respected political leader," Trump tweeted. "His work and voice on so many fronts will be very hard, if not impossible, to replace!"

My warmest condolences to the family and many friends of Congressman Elijah Cummings. I got to see first hand the strength, passion and wisdom of this highly respected political leader. His work and voice on so many fronts will be very hard, if not impossible, to replace! — Donald J. Trump (@realDonaldTrump) October 17, 2019

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Cummings died early Thursday morning "due to complications concerning longstanding health challenges,” his office said. He was 68.

As chairman of the House Oversight and Reform Committee, Cummings was one of the president's most outspoken, powerful and charismatic critics. He held a hearing on the administration's treatment of migrants at the border and memorably chaired a hearing with the president's former personal attorney Michael Cohen Michael Dean CohenAppeals court appears skeptical of Trump's latest argument against tax returns subpoena Judge orders Eric Trump to comply with New York AG's subpoena before Election Day A huge deal for campaign disclosure: Trump's tax records for Biden's medical records MORE.

Cummings was one of the three committee chairmen leading an impeachment inquiry to Trump, though he had been largely absent from the public eye in recent weeks as he battled health problems.

While Trump's tweet upon news of Cummings's death was gracious and respectful, the president spent several days over the summer attacking the congressman and his majority-black district.

The president assailed Cummings as a "racist" and "brutal bully" and accused him of corruption and of failing to address Baltimore's problems. He derided the city of Baltimore, where Cummings grew up and lived, as a "rat and rodent infested mess" where "no human being would want to live."

Cummings responded to the attacks by offering to show Trump around Baltimore to better understand the city and calling it his "constitutional duty" to conduct oversight of the administration.

“Mr. President, I go home to my district daily. Each morning, I wake up, and I go and fight for my neighbors,” Cummings tweeted in July. “It is my constitutional duty to conduct oversight of the Executive Branch. But, it is my moral duty to fight for my constituents.”

The president visited Baltimore in September for a House GOP retreat. He faced protests upon his arrival.

Twenty minutes after his somber message about Cummings, Trump had turned his attention on Twitter back to the ongoing impeachment fight, calling it the "Greatest Witch Hunt in American History!"

Other administration officials and Trump campaign officials also mourned Cummings. Most praised his legacy and offered condolences to his family.

"Sad to learn of the passing of longtime, passionate public servant Rep. Elijah Cummings. May he Rest In Peace. Lifting up his family, friends and colleagues in prayer," White House counselor Kellyanne Conway Kellyanne Elizabeth ConwaySpecial counsel investigating DeVos for potential Hatch Act violation: report George and Kellyanne Conway honor Ginsburg Trump carries on with rally, unaware of Ginsburg's death MORE tweeted.

Sad to learn of the passing of longtime, passionate public servant Rep. Elijah Cummings. May he Rest In Peace. Lifting up his family, friends and colleagues in prayer. — Kellyanne Conway (@KellyannePolls) October 17, 2019

"Everyone woke up to the sad news this morning of the passing of Congressman Elijah Cummings. He was an American icon for sure," tweeted Tim Murtaugh, communications director for the Trump campaign.

"At times like this you put differences aside," Murtaugh added. "Our heartfelt condolences go to his family and friends. Rest In Peace."

Everyone woke up to the sad news this morning of the passing of Congressman Elijah Cummings. He was an American icon for sure. At times like this you put differences aside. Our heartfelt condolences go to his family and friends. Rest In Peace. — Tim Murtaugh (@TimMurtaugh) October 17, 2019

The White House and the U.S. Capitol lowered flags to half-staff on Thursday morning after Cummings' death.

Updated at 9:50 a.m.