PHILADELPHIA — Tim Kaine forcefully criticized Gary Johnson, the Libertarian Party’s presidential nominee, on Thursday after Mr. Johnson equated the targeting of noncombatants by the Syrian government with the accidental bombing of civilians by United States-backed forces.

Mr. Kaine, who has a 26-year-old son in the Marines, said Mr. Johnson’s view was “incredibly disrespectful to the troops.”

“As the father of somebody in the military, I just can’t fathom a comment like that,” Mr. Kaine, a senator from Virginia and the Democratic vice-presidential nominee, told reporters during a campaign stop at an elementary school here.

He went on to lump Mr. Johnson in with Donald J. Trump, the Republican presidential nominee, whom Mr. Kaine and Hillary Clinton, his running mate, have repeatedly declared to be unfit to be commander in chief.