REUTERS/Jonathan Ernst
Mitt Romney.
Romney, the 2012 Republican nominee, turned to Twitter to send his best wishes to the New York businessman.
"Best wishes for our duly elected president: May his victory speech be his guide and preserving the Republic his aim," he wrote.
Romney strongly opposed Trump over the past several months. The former Massachusetts governor even went as far as to give a speech denouncing him and his vision for America.
Trump returned fire, calling Romney a "choke artist" for failing to capture the presidency in 2012.
The Associated Press declared Trump the president-elect at 2:30 a.m. ET, shortly after the wire service placed Wisconsin in his column.
Trump's victory was one of the most stunning political upsets in history. Clinton was strongly favored to emerge as the victor on Tuesday night, with some analysts estimating Trump's odds of winning the election as low as 1%.