How widespread was pacifism in the WW2 USA?

That is a difficult question to answer because one's stance on violence falls within a spectrum, and the cut-offs between regions in the spectrum are not clear cut or well-defined. You could consider Hitler to be a 0/10 on the Pacifist scale and easily get away with it- his aversion to chemical weapons was mostly due to personal experience with them in WWI. Hitler believed in preemptive violence, and his general view of "might makes right, so lets make sure we are mighty" is shared by every patriot of every nation on the planet then and now- far more now than then due to the winner-takes-all nature of ideological and religious bandwagons.

At the opposite extreme one might be tempted to use Ghandi as the exemplar of pacifism, but it would be more realistic to assign him a value of 8 or 9 due to his "goading" of violence from his opponents with the knowledge that the backlash against their violence would be fierce. . A middle-ground score of about 5 could be assigned to Mohammed, as he only believed violence was acceptable for the purpose of defending yourself / your people. The most extreme form of pacifist stance was introduced to the Western world by Jesus Christ himself. In this radical ideology, you are supposed to allow violence of all kinds to be perpetrated against you and can not retaliate. Thats what "turn the other cheek" and "resist not evil" mean. This was provided as a correction for the "eye for an eye" notion of justice that the ancient Jews found wise. Jesus seemed to think that though that was a step in the right direction, people would become over-zealous in pursuing equitable justice in that fashion and it would lead to escalation. His stance was instead the most likely to de-escalate any conflict. The fact that a mere 4 centuries after he died the Church of England made burning people alive a punishment for not believing in the Holyness of Jesus is fairly ironic. Especially when you consider the most common victims were His blood relatives.

In any case; there is a huge spectrum of possible beliefs on the use of violence; and they range from practical to philosophical or religious in motivation.

I would argue that the 4 million KKK members in America during WWII suggests that actual support of Nazism only died out vocally during he war, but remains a part of our national identity even now. "Anti-racial mixing" laws were a staple of Nazi thought. In the USA, such laws finally left the constitutions of 12 states in the late 90s to 2001, even though they were deemed unconstitutional in 1965. Its not like Hitler invented ethnic cleansing or his anti-pacifist stance, both of which were patently Christian since militant Romans adopted (and took over) the Christian faith. Hitler just made a more extreme example, and modernized the practice of genocide. The Jews were an easy target because they were small in number, and being largely pacifists during WWI made them the "dagger in the back" that Germans could blame for their prior failure to achieve military supremacy.

Remember that 97.5% of Germans were Christian in the 1939 as revealed by their census data (source: wikipedia.org/wiki/Gottgläubig). You would think that this would have meant somebody could have mentioned that loving your enemy includes not putting them in gas chambers, and that "resist not evil" and "turn the other cheek" are contrary to the "Gott Mit Uns" ("God's with US") slogan that was imprinted on the standard Nazi belt buckle. But then, somebody could be reminding their fellow Americans of the same thing right now even as our nation decides which Muslim nation would be best to war upon. [source: https://www.washingtonpost.com/news/the-fix/wp/2014/09/29/the-american-people-are-war-weary-they-are-far-from-pacifists/]

I find it interesting how little people care about the stances of their figureheads and saviors. For instance Pope Francis is the first Pope ever to endorse pacifism publicly, just as Jesus did. However,

Seven months after the pope’s speech to Congress, Michael Sean Winters of the National Catholic Reporter commended the witness of Catholic pacifists but concluded that “just war theory still reflects the demands of justice in an often brutal world.” Earlier, in 2009, the Catholic writer Austen Ivereigh bluntly stated that followers of Christ “must be willing to wage war in defence of the oppressed,” among other reasons. (source: https://www.americamagazine.org/politics-society/2019/09/25/nine-decades-ago-pacifism-was-called-un-american-are-attitudes)