SCRANTON, Pa. — Vice President Joseph R. Biden Jr. warned on Monday that Donald J. Trump, even as a candidate, had elevated the dangers confronting American allies and military personnel overseas.

Mr. Biden said that on a trip to Kosovo and Serbia this week, he would be compelled to “reassure” allies that the United States would honor its commitment to NATO, given Mr. Trump’s comments saying he would reassess the arrangement if elected.

“Because they’re worried,” Mr. Biden said, in his first campaign appearance beside Hillary Clinton.

He added that if his son Beau, who died of brain cancer last year, were still serving in Iraq, he would advise him that the danger had increased “a couple clicks” because of Mr. Trump’s remarks, which have included the suggestion that President Obama was the “founder” of the Islamic State.

“This guy’s shame has no limits,” he said.

Mr. Biden suggested Mr. Trump had demonstrated a fondness for brutal leaders like Saddam Hussein — “He would have loved Stalin,” Mr. Biden said twice — and repeatedly reminded the crowd of the stakes.