At times, The Catholic Knights seem to have a more conservative view on politics than does even the Pontiff himself.
Both Biden and Kaine before him are Catholics. Kaine was a pretty devout Jesuit. I don't think the Midwest is in quite as much peril this election cycle as you think. The battlegrounds will likely be Arizona, Florida, and North Carolina.
I will firstly say that Harris never called the Knights of Columbus an extremest group from any transcript I can find. The Senator from Hawaii, Hirono did say,
"You reported that you have been a member of the Knights of Columbus since 1993. The Knights of Columbus has taken a number of extreme positions. For example, it was reportedly one of the top contributors to California’s Proposition 8 campaign to ban same-sex marriage. If confirmed, do you intend to end your membership with this organization to avoid any appearance of bias?"
I think this letter to the editor of the LA Times sums up my opinion on the matter pretty well...
To the editor: Rabbi Mitchell Rocklin calls out Sens. Kamala Harris (D-Calif.) and Mazie Hirono (D-Hawaii) for questioning whether a judicial nominee who is a member of the Knights of Columbus can serve on the federal bench. Rocklin calls their concern a constitutionally prohibited religious test, but it is not.
The senators do not claim that nominee Brian Buescher is unfit to serve on the bench because he is Roman Catholic. Their concern is whether his membership in the Knights of Columbus may affect his ability to judge impartially.
Roman Catholicism is a religion. There is no religion called Knights of Columbus, which is in fact an organization that takes political positions that should be of concern to the Senate.
It is entirely appropriate for senators to query a judicial nominee about his membership in an organization that advocates elimination of both marriage equality and a woman’s right to choose before they reward him with a lifetime appointment to the federal bench.
I.E. if someone was asking a person of Ismalic faith skeptically questioning their previous membership in a conservative Islamic group which had fundamentalist doctrinal opinions about some beliefs of the faith., that would be different than questioning their belief in Islam as a whole.