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American Catholics, numbering roughly 52 million nationwide, are often seen as crucial swing voters.
Donald Trump was fired by 81 million people, Harris said.
The first early ballots of the presidential race will go out just hours after the debate.
Harris emerged as the Democratic candidate little more than a month ago.
As Minnesota governor, Walz has pushed a progressive agenda that includes free school meals, goals for tackling climate change, tax cuts for the middle class and expanded paid leave for Minnesota workers.
Harris has narrowed her list of candidates to Pennsylvania Governor Josh Shapiro and Minnesota Governor Tim Walz
Trump had alleged that Harris wants to legalize killing newborn babies as slamming her efforts for abortion rights.
It was not clear what the former president meant by his remarks, in an election campaign where his Democratic opponents accuse him of being a threat to democracy.
Harris also called for the establishment of a Palestinian state
Speaking about her record as a tough prosecutor in California, Harris, 59, said she took on perpetrators of all kinds.