KANSAS CITY, Mo. — President Trump transformed a speech on Tuesday before thousands of veterans into a political rally, summoning a Missouri Senate candidate to the stage, venting about Democratic lawmakers and slamming the news media for not upholding his preferred interpretation of the success of his policies.

“Stick with us. Don’t believe the crap you see from these people, the fake news,” Mr. Trump said at the annual convention of the Veterans of Foreign Wars, as part of the crowd booed and hissed in the direction of the press corps.

“What you’re seeing and what you’re reading is not what’s happening,” the president added.

A spokeswoman for the veterans group, which is nonpartisan, said in a statement after the event that organizers were “disappointed to hear some of our members boo the press.”

The event on Tuesday was intended to showcase one of the Trump administration’s top priorities: improving the care of the nation’s veterans and stabilizing a department demoralized by scandal and staff vacancies.