Vice President Kamala Harris pledges to unite the Democratic Party. Also, New York's Democratic delegates vote to support Harris. Plus, Harris holds her first presidential campaign rally in Wisconsin. And the Harris candidacy reshapes key Senate race strategies. Rep. Joyce Beatty, Gov. Kathy Hochul, Ben Wikler, and Sen. Gary Peters join Lawrence O'Donnell.

Pennsylvania leads the U.S. in political ad spending. Also, former national security leaders endorse Vice President Harris. Plus, Manhattan District Attorney Alvin Bragg urges Judge Juan Merchan to reject Donald Trump's bid to toss his conviction. And gun safety group March for Our Lives makes a historic endorsement. Sen. Bob Casey, Amb. Susan Rice, Andrew Weissmann, and David Hogg join Lawrence O'Donnell.