Republican candidate Wendell Willkie waged a nasty campaign against President Franklin D. Roosevelt in 1940 before they became political allies in the leadup to World War II.

Willkie called FDR’s decision to seek a third term a dangerous precedent, warning voters that if the Democratic incumbent won, they would be “serving under an American totalitarian government before the long third term is finished.”

Roosevelt backers returned the favor, with New York Democratic Senator James Mead dismissing Willkie as a “self-appraised miracle worker who has never demonstrated any qualifications for leadership in government.”

But that animus quickly evaporated after Roosevelt’s decisive victory, and FDR invited his former opponent to the White House. As FDR’s Labor Secretary Frances Perkins recounted in her book, The Roosevelt I Knew, the president was surprised that he took a liking to Willkie.

“You know, he is a very good fellow,” FDR told her. “He has lots of talent. I want to use him somehow. I want to offer him a post in the government.” Roosevelt said he wasn’t envisioning Willkie as a cabinet member. “I want him to do something where the effort is nonpolitical but important. But I’d like to use him, and I think it would be a good thing for the country, it would help us to a feeling of unity.”

During the 1940 election, Roosevelt had promised to keep the United States out of World War II, while Willkie predicted the president would drag the country into it. At the time, many Americans were leery of getting involved in another war, just 22 years after the end of World War I.

But Willkie, a registered Democrat until 1939, was much more of an internationalist than many of his fellow Republicans who took an isolationist view.

In December 1940, British Prime Minister Winston Churchill sent a message to FDR that his country was running out of money to buy weapons—putting Great Britain in mortal danger from the German onslaught. FDR enlisted Willkie as an emissary to personally deliver a letter to the British leader.

“Dear Churchill, Wendell Willkie will give you this—He is truly helping to keep politics out over here,” FDR wrote in the letter, which was dated January 20, 1941, the day of his third inaugural. “I think this verse applies to your people as it does to us.” FDR then quoted from a Longfellow poem, "The Building of the Ship:”

Sail on, Oh Ship of State!
Sail on, oh Union strong and great.
Humanity with all its fears
With all the hope of future years
Is hanging breathless on thy fate.

Churchill was said to have found the letter to be “an inspiration” and had it framed, according to the Library of Congress.

That same month, Democrats introduced FDR’s Lend-Lease bill in Congress, which would authorize the U.S. to lend or lease war supplies to any nation deemed "vital to the defense of the United States”—namely, Britain.

It was savaged by some isolationists in both parties, such as Senator Burton K. Wheeler, a Montana Democrat, who said the plan would “plough under every fourth American boy.” Senator Arthur Vandenberg, a Michigan Republican, claimed the bill would give FDR the authority “to make war on any country he pleases any time he pleases.”

In its news coverage, the conservative Chicago Tribune routinely referred to it as the “dictator bill.”

It was in this fraught political atmosphere that Willkie, back from his trip to Great Britain, testified at a Senate Foreign Relations Committee hearing in February packed with 1,200 people. The House had already passed the bill.

At the hearing, Democratic Senator Bennett Champ of Missouri, asked Willkie about his contention during the campaign that FDR would lead the country to war.

“I struggled as hard as I could to defeat Franklin Roosevelt, and I tried to keep from pulling my punches,” Willkie replied. “He was elected president. He is my president now.” According to Jean Edward Smith’s 2008 biography, FDR, spectators in the room greeted the comment with thunderous applause.

North Dakota GOP Senator Gerald Nye followed up, asking Willkie if he still believed in his prediction that an FDR victory would mean the United States would be at war by April 1941. Willkie dismissed the line as a “a bit of campaign oratory.” He added, “I’m very glad you read my speeches, because the president said he did not.” The room erupted in “howls of laughter and sustained, foot-stomping applause,” Smith wrote.

The Senate passed the bill 60-31 in March, and it proved a vital lifeline for Britain in the months before the United States entered the war in December, 1941.

Willkie ran for president again in 1944, but dropped out after losing the Wisconsin primary to the GOP’s eventual nominee, Thomas Dewey. (Willkie died a few months later, in October, 1944.)

As World War II was nearing its end, FDR privately dressed down his closest aide, Harry L. Hopkins, for making a negative comment about Willkie.

“Don’t ever say anything like that around here again. Don’t even think it,” FDR told Hopkins. “You of all people ought to know that we might not have had Lend-Lease or Selective Service or a lot of other things if it hadn’t been for Wendell Willkie. He was a godsend to this country when we needed him most.”