In the 1940 election, FDR's popular vote was nearly the same as in 1936. However, his electoral vote total fell off considerably. How do you...

To understand the results of this election properly, we first have to understand how President Franklin D. Roosevelt did in the popular vote in each of these elections.  Your question says that his popular vote in 1940 was “nearly the same” as in 1936.  In a sense, this is true.  In 1936, FDR received 27.7 million votes.  In 1940, he received 27.3 million votes.  This means that something in the range of 400,000 fewer people voted for him in 1940 than in 1936.  This qualifies as “nearly the same.”  However, we also need to look at the percentage of the popular vote that FDR won in each of these years.  These numbers tell us a much different story.  In 1936, FDR won just over 60% (60.8%) of the popular vote.  By 1940, that had declined dramatically to 54.7%.  This does not count as “nearly the same.”  So, while FDR’s vote totals were nearly the same, the percentage of the popular vote that he won was most definitely not.  This explains why FDR did not win as many states in 1940 as he did in 1936.

If FDR’s total popular vote remained almost the same but his percentage dropped, many more people must have voted.  In 1936, about 44.4 million people voted for either FDR or his Republican opponent, Alf Landon.  In 1940, that number had increased to about 49.5 million.  This means that about 5 million more people voted in 1940 than in 1936.  This increase can be explained in large part by looking at the overall population of the US in those two years.  The US population in 1940 was about 9.4 million people higher than it was in 1930, thus explaining some of the increase in voter turnout.

This all explains why Roosevelt won fewer electoral votes while winning the same number of popular votes.  But why did a lower percentage of people vote for FDR in 1940?  There are two main reasons.  One reason is that Wendell Willkie, the Republican, charged that Roosevelt was secretly trying to get the US involved in WWII. This was a very unpopular idea in the US as isolationism was strong in those days.  The more important reason is that Roosevelt was breaking tradition by being the first president ever to run for a third term.  Even George Washington had chosen not to run for a third time and many people felt that it was wrong for Roosevelt to do this.  This is the most likely reason why FDR won a smaller percentage of the popular vote in 1940.