Mitt Romney’s political assault on Donald J. Trump on Thursday was so savage that historians strained to recall any precedent in American politics, with a major party’s former nominee blistering his party’s leading presidential candidate in such a personal and sweeping fashion.

Al Smith, who in 1928 was the governor of New York and the Democratic presidential nominee, later turned on Franklin D. Roosevelt during the New Deal and warned, much as Mr. Romney did on Thursday, that Roosevelt’s liberal policies would lead toward totalitarianism. In a high-profile 1936 speech before the American Liberty League, an anti-New Deal group, Smith offered a “solemn warning” that Americans must realize “there can only be one capital – Washington or Moscow.”

“There can only be one atmosphere of government, the clear, pure, fresh air of free America or the foul breath of communistic Russia,” Mr. Smith said. “There can be only one flag, the Stars and Stripes, or the red flag of the godless union of the Soviet.”