My data is taken from opinion polls published in various issues of Public Opinion Quarterly from 1937 to 1944. Even before the war in Europe started, most Americans thought it would draw us in if it happened. They didn't like the idea, and certainly there was little sympathy for the Jews, who were listed as one of the most disliked groups in America in several open-response polls.
But after France fell, American opinion began to shift. We still wanted to stay out, but if given the choice between letting Germany defeat Britain and directly entering the war in some capacity, the numbers moved steadily towards "help Britain." Although there were fewer polls about Japan, one taken just before Pearl Harbor found a majority of Americans agreeing with sanctions on Japan, even if it meant war. So the public was somewhat prepared to fight if given a good strategic reason to do so (stopping Japanese expansion or saving Britain). But of course, we didn't actually go to (open) war against Germany until Hitler declared war against us.
Intriguingly, just a few weeks after Pearl Harbor, the public was surveyed about which country they thought was a bigger threat to the United States. Two-thirds of them picked Germany over Japan -- either showing a great deal of strategic calculation, or just following elite opinion, which had already decided on a Germany-first strategy.
One final poll deserves mention. In 1944, the public was asked whether they believed that Jews were being exterminated:
December 1944: Do you believe the stories that the Germans have murdered many people in concentration camps are true or not true?
Not true 12%
No opinion 12%
True 76%
Of those who said true: Nobody knows, of course, how many may have been murdered, but what would be your best guess?
100,000 or less 27%
100,000 to 500,000 5%
500,000 to 1,000,000 1%
1,000,000 6%
2,000,000 to 6,000,000 8% (the correct answer at the time)
6,000,000 or more 4%
Unwilling to guess 25%
The straight answer is that after the Japanese attack on Pearl Harbor the US declared war on Japan and Japan only the next day. Relations with Germany were on the brink of declared war, and the US was firmly in support of the Allies but didnt have the cause and FDR lacked the political capital to include Germany from the start despite his desire to.
Germany however solved that problem for the US. On December 11, 1941 Germany declared war on the US in response it said to its numerous violations of neutrality, and in solidarity with its Japanese allies against a supposedly weak willed and unorganized foe.
The US been inching its way towards more and more direct aid to the Allies and especially Britain. FDR saw Germany as the greatest threat to world peace, and was committed to doing what he could to stop Hitler's violent expansionism. While their brutal repression of many groups was widely reported on it should be recalled that there were large segments of the US population that were not fans of groups like the Jews, Poles, Slavs, and Romani to begin with and public opinion on the war was slow to shift.
But far more overt was the impact of repeated agreements with Britain. Lend Lease which began in Spring 1941 is the most famous allowing financing and delivery of sorely needed military supplies for British use or shipment to Russia following their invasion. Building on the earlier Cash and Carry policy and FDR's attempt to shift the nation from outright Isolationism to at least a middle ground of working to contain aggressor nations and isolate them by embargo and any other efforts. Destroyer's for Bases gave the US base usage in the Western Hemisphere in return for several dozen WW1 vintage ships to the Royal Navy to help fight the U-Boats. The US also agreed to send in the Marines to occupy Iceland to allow the redeployment of the British garrison and keep it from German hands. While the Atlantic Charter in the summer of 1941 and secret military talks prior to that confirmed that the US was firmly against any continuing Nazi Empire, and laid the groundwork even for the UN and the post war world. The famous "Germany First" policy to prioritize Europe over the Pacific had been recognized in the US even before the invasion of Poland. The famous Rainbow Plans were drawn up in the Summer of 1939.
Rainbow 5 being;
RAINBOW 5 assumed the United States, Great Britain, and France to be acting in concert; hemisphere defense was to be assured as in RAINBOW 1, with early projection of U.S. forces to the eastern Atlantic, and to either or both the African and European Continents; offensive operations were to be conducted, in concert with British and allied forces, to effect the defeat of Germany and Italy. A strategic defensive was to be maintained in the Pacific until success against the European Axis Powers permitted transfer of major forces to the Pacific for an offensive against Japan. http://www.history.army.mil/books/70-7_01.htm
While the Plan DOG Memo in mid 1940 would double down on this being the best possible option for an assured victory leading to a global return to peace.
While all were certainly calculated and provocative and inched the nations closer to war one additional spark must be mentioned, that of the Neutrality Patrols. In Summer of 1941 FDR ordered the Navy to detain or sink any warship belonging to a power not local to the hemisphere found in the Western Atlantic. Ostensibly to keep the war from spreading it was really meant to create an off limits area for the U-Boats following US flagged vessels starting to be caught in the battle just as in WW1. While US and Canadian ships would escort convoys to a halfway point and turn them over to British vessels. A trio of battleships was even redeployed from the Pacific should any of the larger German warships try anything. This did lead to several engagements and losses on both sides and most famously the USN destroyer Reuben James was sunk on October 31, 1941 off Iceland protecting covering a Convoy. So arguably 6 weeks later the deceleration of war was in essence the logical progression of rising tensions. And greatly simplified things for the Allies having the US and UK fighting the same foes the world over.