"The recent attacks upon (McMaster) from the so-called 'alt-right' are disgraceful. Since this fringe movement cannot attract the support of decent Americans, it resorts to impugning the character of a good man and outstanding soldier who has served honorably in uniform and sacrificed more for our country than any of his detractors ever have," McCain said in a statement Monday afternoon. "Such smear tactics should not be tolerated and deserve an emphatic response."
"I hope the President will once again stand up for his national security adviser and denounce these repugnant attacks, which arise from the same purveyors of hatred and ignorance who precipitated the recent violence in Charlottesville," McCain added.
McCain's statement came as far-right leaders pushed new lines of attack against McMaster, which ranged from claims that he's not a strong enough supporter of Israel to unfounded allegations that McMaster has a drinking problem -- the latter promulgated by the far-right blogger Mike Cernovich.
The smear campaign is the latest effort by McMaster's adversaries to undermine the national security adviser's credibility as he has feuded internally with White House chief strategist Steve Bannon
, particularly over the war in Afghanistan. Bannon has privately denied any involvement in the anti-McMaster efforts, according to officials.
"The President directly expressed his support for Gen. McMaster from Bedminster last week," White House press secretary Sarah Huckabee Sanders said when asked if Trump had any additional comment to offer.
Trump last week expressed confidence in his national security adviser while taking questions at his golf course in Bedminster, New Jersey, saying he "absolutely" had confidence in him.
"He's our friend. He's my friend. And he's a very talented man. I like him and I respect him," Trump said then.
GOP megadonor Sheldon Adelson also sought to distance himself from the anti-McMaster campaign.
After a pro-Israel group Adelson funds slammed McMaster, Adelson representative Andy Abboud said the casino magnate is in no way connected to the campaign against McMaster.
"He doesn't support the campaign against McMaster. He has heard from many people that McMaster is good on many Israeli issues," Abboud told CNN.
Abboud added that Adelson hasn't spoken with Morton Klein, the national president of the Zionist Organization of America, which issued a report last week arguing McMaster has not been a strong enough supporter of Israel.
"He hasn't spoken to Mort Klein about it," Abboud said, adding that the report "has nothing to do with Mr. Adelson."