Veterans groups are calling on President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE to lower the White House flag to half-staff to honor Sen. John McCain John Sidney McCainAnalysis: Biden victory, Democratic sweep would bring biggest boost to economy The Memo: Trump's strengths complicate election picture Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' MORE (R-Ariz.), who died on Saturday night of brain cancer.

“It’s outrageous that the White House would mark American hero John McCain’s death with a two-sentence tweet, making no mention of his heroic and inspiring life,” Joe Chenelly, the executive director of veterans advocacy group AMVETS, said in a statement.

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"And by lowering flags for not one second more than the bare minimum required by law, despite a long-standing tradition of lowering flags until the funeral, the White House is openly showcasing its blatant disrespect for Senator McCain’s many decades of service and sacrifice to our country as well as the service of all his fellow veterans," Chenelly added.

The American Legion, the country's largest wartime veterans service organization, joined in condemning Trump for his muted response to McCain's death. McCain was a member of the American Legion and retired from the U.S. Navy at the rank of captain.

"On behalf of the American Legion's two million wartime veterans, I strongly urge you to make an appropriate presidential proclamation noting Senator McCain's death and legacy of service to our nation, and that our nation's flag be half-staffed throughout his internment," Denise Rohan, the national commander of the American Legion, said in a statement.

Flags at the White House returned to full-staff on Monday, less than 48 hours after they were lowered.

Trump had an adversarial relationship with McCain and continued to mock him as his condition debilitated in the final months of his life. The president reportedly nixed putting out a statement praising McCain, according to The Washington Post.

He instead reportedly told his aides he preferred to tweet his condolence, posting a pared-down statement in which he did not offer praise for the Arizona Republican.

"Traditionally, the death of a sitting United States senator would be met with a presidential proclamation and flags flying at half-staff throughout the country until the funeral of the deceased," AMVETS said in a statement. "This follows national tradition, as shown after the deaths of Senators Ted Kennedy and Robert Byrd."

"But John McCain was not just a sitting senator," the statement said. "He was a war hero, twice a presidential contender, and a national treasure who devoted his entire adult life to protecting and improving the American way of life."

McCain was held captive as a prisoner of war in Vietnam for more than five years, and spoke out on behalf of the military and veterans throughout his life.

Trump will not attend McCain's funeral, a spokesperson for the late senator confirmed to the Post on Monday.