WASHINGTON — An awkward handshake is really the least of their worries.

As President Trump continues to rage against impeachment — and the Democrats and whistle-blower he holds responsible for bringing it about — visiting world leaders are encountering a different kind of diplomatic mission.

It includes a welcome ceremony, a meeting with Mr. Trump and an invitation to sit stone-faced for an indeterminate amount of time on live television as the president accuses people of treason, lies and corruption. And sometimes the session is reprised a little later in a formal news conference.

That was what happened on Wednesday when President Sauli Niinisto of Finland became the latest foreign leader to strike a straight-lipped contrast to Mr. Trump as Mr. Trump defended himself and attacked his adversaries. Not once but twice.

As reporters crowded into the Oval Office, Mr. Trump sat beside his guest and accused Democratic lawmakers, including Representative Adam B. Schiff, the chairman of the House Intelligence Committee, of treason. Mr. Trump also suggested that the congressman was not fit to carry the secretary of state’s “‘blank’ strap,” as Mr. Niinisto looked on.