Public sentiment changed greatly as events progressed in Europe from 1938 to 1941 and tended to be out in front of the actions of the President and Congress on support for the Allies until Pearl Harbor. FDR was certainly a driver of public support for the Allies in his speeches and fireside chats, but after the 1938 elections tended to act only when he was sure he was on firm political ground.
Isolationism in the US during this period was complicated. The largest formal isolationist group was the America First Committee which was started by a group of Yale students and spread throughout the Ivy League before being co-opted by a group of midwestern businessmen. The America First Committee was eventually destroyed from within by an antisemitic speech given by Charles Lindbergh in Des Moines on September 11, 1941, that turned the nation against them. While America First was the largest isolationist group, the staunchest isolationists groups were the American Bund and the Communist Party of the United States of America who held daily marches against entering the war outside the White House until June 1941. The disparity of groups in the isolationist movements makes it hard to speak about as a public movement. It’s easier to talk about isolationism in Congress, although it was still a wide variety of motivations including republicans, pacifists, and democrats upset with FDR over the 1938 elections.
Although all isolation debates generally involved keeping the US out of another war in Europe, the first issue was whether the US should sell armaments to France and Britain. The export of “arms, ammunition, and implements of war” from the US to any foreign country at war was illegal under the neutrality acts of 1935 and 1937, although the 1937 act allowed export of food and fuel to belligerents. After the German invasion of Czechoslovakia and much debate, FDR managed to pass the Neutrality Act of 1939 which allowed the sale of arms to nations under war under the terms of “cash and carry” although it did not repeal the Johnson Debt Default Act which forbade loaning money to countries that had defaulted on their WW1 debt. The other side of that coin was that the 1939 act prohibited sending goods to nations at war on US ships.
The next debate centered on providing arms to Britain and France on terms other than “cash and carry.” While private banks were still prohibited from making loans to Britain and France, the Lend Lease Act which was signed in March of 1941 allowed the US government to loan military equipment to any belligerent nation at the discretion of the President. This was strongly supported by the US public at the time and was necessary as Britain had begun to run out of US dollars and gold that could be used to purchase under the existing terms. Even after lend-lease, private banks were still prohibited from lending money to Britain by the Johnson Debt Default Act, although most wouldn’t have considered it a safe investment at the time.
The final debate was about direct intervention in the war. This is the one point that was generally agreed upon by all Americans, and the consensus opinion before Pearl Harbor was that US troops should not be sent to fight in Europe. When opinion polls asked whether US troops should be sent if Britain and France were about to fall, this number rose from 16% in 1939 to 44% after the invasion of Poland. The response to whether the US should send troops “now” was always much lower, only rising to 7% after the fall of France. This only changed after the attack on Pearl Harbor.
Circling back to public opinion, the US public were greatly affected by the events in Europe. The Munich Crisis, the invasion of Czechoslovakia, the invasion of Poland, and the fall of the Lowlands and France were all front-page news in the US and very important to a changing public opinion. Selling arms to Britain and France became popular after Czechoslovakia and more popular after Poland. Providing support to Britain at any cost became popular after the fall of France. Sending troops to fight in Europe was never popular before Pearl Harbor.
Sources:
Langer & Gleason (1952). The Challenge to Isolation 1937-1940
Olson, Lynne (2013). Those Angry Days