The first — political fallout from a fatal stabbing attack on London Bridge last week — may be manageable because the circumstances preceding the incident are complex and involve both parties.

But the second — Donald Trump, who is in London for a summit meeting of NATO leaders that begins today — may prove harder to contain. Mr. Johnson’s close ties to Mr. Trump are a liability in an election, and there is little he can do to control a president our reporters call “a wild card who could jolt the election with a single tweet.”

Details: The White House says Mr. Trump’s visit will include one-on-one meetings with President Emmanuel Macron of France and Chancellor Angela Merkel of Germany — but not Mr. Johnson. It is unusual for the U.S. president not to participate in a bilateral meeting with the leader of NATO’s host country.

Attack victims: The two victims of last week’s knife attack in London were bright, idealistic young people who believed passionately in prison rehabilitation.