“the biggest single scheme” The largest Nazi operation aimed at the 1940 U.S. presidential election.
But there is an important distinction between $5 million allocated and $5 million actually spent.
The $5 million Davis–Göring operation
In 1940, American oilman William Rhodes Davis went to Germany and met Hermann Göring. Davis proposed using American political connections to defeat Roosevelt or otherwise prevent his reelection. The plan initially envisioned John L. Lewis, the powerful isolationist labor leader, as a possible political vehicle. Göring reportedly made about $5 million available, with funds held at the German Embassy in Washington. (HNN)
The intended operation included:
- Trying to prevent Roosevelt's renomination by influencing Democratic convention delegates.
- Supporting an isolationist candidate who could defeat Roosevelt.
- Using Lewis's political influence against Roosevelt.
- Financing anti-Roosevelt propaganda and political activity.
- Spending money against Roosevelt allies, including an effort involving Pennsylvania Senator Joseph Guffey. (Wikipedia)
The plan ran into a major problem: Roosevelt was overwhelmingly renominated, and Republicans chose Wendell Willkie, who was himself an internationalist rather than the isolationist candidate Davis wanted. Consequently, the operation couldn't produce the political outcome Germany had hoped for. (HNN)
The really important caveat
We cannot honestly say that Hitler spent $5 million to elect Willkie.
The evidence establishes that approximately $5 million was allocated/available for the operation, but historians don't have a complete accounting showing where all of it went. Contemporary and later investigations produced substantially smaller figures for identifiable expenditures. A 1941 Congressional Record discussion, for example, referred to Davis bringing $440,000 of Nazi money into the United States. (Congress.gov)
And the Lewis radio broadcast is particularly interesting: Davis arranged financing for Lewis's October 23, 1940 attack on Roosevelt and endorsement of Willkie, without Willkie's knowledge. (Wikipedia)
So I'd characterize it this way:
The largest documented Nazi financial commitment to a single U.S. election operation was the approximately $5 million Göring/Davis fund intended to influence the 1940 presidential election. It was a serious covert foreign-interference scheme, but most of the $5 million was apparently never demonstrably spent on the election, and there is no evidence it changed the result.
There is also a fascinating piece of evidence from the German Embassy's own personnel that describes this as a major operation and discusses the $5 million figure. (Scribd)
If you're asking because you've seen the claim that “Hitler secretly gave the Republican Party $5 million in 1940,” that's a more complicated claim—and “gave the Republican Party” is not an accurate description of what the primary evidence establishes.