In seeking to capture his ideals, Mr. Trump has embraced the slogan “America First.” An echo of Charles Lindbergh and Pat Buchanan, the phrase evokes an old-fashioned American nationalism — tinged with nativism, a suspicion of outsiders and a retreat from foreign entanglements. The third night of Mrs. Clinton’s Democratic National Convention, heavy on national security themes, could have been titled “America First in the World.” Democrats celebrated the country’s diversity, with Senator Tim Kaine of Virginia, the vice-presidential nominee, ladling on the Spanish. Prominent Democratic hawks, including Leon Panetta, a former C.I.A. director and secretary of defense, made the case for internationalism and multilateral engagement, harking back to the Cold War tradition.