FDR was the first and only person to serve more than two terms as President of the United States. His presidency, many would say, led to the passing of the Twenty-second amendment of the constitution. What was the immediate reaction to FDR seeking a third term? Was there major backlash?
There was some backlash, but I wouldn't call it major as he still won fairly easily. After winning his first two terms with 57.8% and 60.8% of the vote respectively, he won his third term with 54.7%.
It was an issue though, and it's partially the reason why the New York Times did not endorse him in 1940 despite endorsing him the previous two times. In their editorial, they listed four reasons to not support FDR, with the fourth being "The Third Term Issue":
The doctrine of one man's indispensability is a new doctrine for this country. It is a doctrine which less scrupulous men in Europe have used to root themselves in power. It is a doctrine which we in the United States have good reason to question...
http://graphics8.nytimes.com/packages/pdf/opinion/timeline/willkie-1940.pdf