Klobuchar (and Harris for that matter) would have a hard time winning progressive/African American votes because of their past as DA's/Attorney Generals. While I don't forsee a third party challenge, Klobuchar winning the nomination means she would have tacked hard to the left on social justice issues, and probably gone full defund the police to compensate for her record as Hennepin County DA (Harris likely will have to do the same should she run in 2024 or 2028).
Senator Amy Klobuchar (D-MN)/Senator Cory Booker (D-NJ) 50%
President Donald Trump (D-NY)/Vice President Mike Pence (D-IN) 48%