Washington Post conservative columnist Jennifer Rubin on Monday said “there are serious concerns” about 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’s “mental stability.”

“There are serious concerns about this president’s mental stability, and there are serious problems when you have a president of the United States who is so erratic, so unreliable. This is incredibly damaging,” Rubin told CNN’s “New Day.”

Rubin pointed to the discrepancies between Trump’s own statements on Twitter and the messages disseminated by his White House press shop.

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“The problem is that we have a president who is not focused on the right things, who created controversy, who has taken the worst possible opportunity to attack the mayor of a city that has been attacked by terrorists,” Rubin said.

Trump over the weekend criticized London Mayor Sadiq Khan after attacks in the United Kingdom’s capital city left seven people dead and dozens wounded.

“At least 7 dead and 48 wounded in terror attack and Mayor of London says there is ‘no reason to be alarmed!’ ” Trump said early Sunday.

Rubin also described Trump as “the world’s worst client or the world’s worst witness,” noting the president often strays from the controlled messages crafted by his staff.