A criminal violation of the foreign contribution ban occurs when a person accepts the illegal donation “knowingly and willfully.” Mueller wrote that it would be difficult to prove that Trump Jr. took the meeting with the Russian lawyer knowing it was illegal.

Trevor Potter, a former chairman of the Federal Election Commission who has advised Republican presidential campaigns, said Trump should understand that the Mueller investigation and the experience of the past two years would mean that prosecutors will assume he and his campaign aides now understand the law and would be more likely to assess that any violations of the foreign contribution ban in the future were made knowingly.

One close adviser to the White House said there were two key reasons for Trump’s comments: He would never concede that his campaign did anything wrong, and he did not want to implicitly criticize Trump Jr., who had testified on Capitol Hill that day…

“Usually there is a competition among presidential candidates to see who can be toughest on our adversaries,” Jacobs said. “Here, the president is openly welcoming the assistance of foreign powers.”