The Democrats visited Wisconsin and Michigan, hoping to shore up support among the younger, diverse, labor-friendly voters who were instrumental in helping President Joe Biden win the 2020 election.
Harris told the day's first rally in Eau Claire, "As Tim Walz likes to point out, we are joyful warriors." Contributing to that feeling, the Harris campaign said it had raised $36 million in the first 24 hours after she announced Walz as her running mate.
The vice president said the pair look on the future with optimism, unlike former President Donald Trump whom she accused of being stuck in the past and preferring a confrontational style of politics - even as she criticized her opponent herself.
"Someone who suggests we should terminate the Constitution of the United States should never again have the chance to sit behind the seal of the United States," Harris said, her voice rising amid applause from a crowd her campaign said numbered more than 12,000.
Wednesday's campaign swing was especially important for her and Walz since Biden's winning coalition from four years ago has showed signs of fraying over the summer - particularly in Michigan, which has emerged as a focal point of Democratic divisions over Biden's handling of the Israel-Hamas conflict.
With Biden now out of the race, leaders of the Arab American community and key unions say they are encouraged by the running mate choice. Walz's addition to the ticket has soothed some tensions, signaling to some leaders that Harris had heard concerns about another leading contender for the vice presidential slot, Gov. Josh Shapiro of Pennsylvania, who they felt had gone too far in his support for Israel.
"The party is recognizing that there's a coalition they have to rebuild," said Abdullah Hammoud, the mayor of Dearborn, Michigan. "Picking Walz is another sign of good faith."
Trump, too, has put emphasis on appealing to voters in Midwestern states with his choice of Vance an Ohio Republican senator, as his running mate. Vance was even bracketing the Harris-Walz ticket with Michigan and Wisconsin appearances of his own on Wednesday.
The dueling schedules overlapped enough that while Harris was still greeting a group of Girl Scouts who came to see her arrive at Chippewa Valley Regional Airport in Wisconsin, Vance's campaign plane landed nearby and was taxiing in the distance.
Harris posed for a group picture with the girls around the same time Vance was deplaning, and he began walking over to Air Force Two, trailed by his security detail.
The vice president eventually climbed into her motorcade, and it pulled away before they could interact. Still, that the pair came so close to doing so on a tarmac was unusual given the carefully scripted nature of campaign schedules.
"I just wanted to check out my future plane," Vance later told reporters, meaning that he'd travel on Air Force Two should he and Trump be elected in November. He also criticized Harris for not taking questions from reporters, though she sometimes answers shouted questions while boarding or leaving her plane for campaign stops.
Vance later told the crowd at his Eau Claire event, "We actually just saw the vice president's plane" and then joked of reporters traveling with him, "I figured they must be lonely because Kamala Harris doesn't take any questions."
"If those people want to call me weird I call it a badge of honor," Vance said, responding to a moniker Walz used to describe him that made the Minnesota governor notable online in the days before Harris tapped him as her running mate.
Addressing the Democrats' Wisconsin rally ahead of Harris, Walz had some critical words for Vance but trained most of his sharpest words on Trump, saying the former president "mocks our laws, he sows chaos and division amongst the people and that's to say nothing of the job he did as president."
Republicans are trying to portray Harris and Walz as too liberal for the Midwest, with Sen. Ron Johnson, R-Wis., saying on a conference call that Walz is "part of the radical, crazy left as is Vice President Harris."
But Democratic enthusiasm has surged since Harris announced her candidacy and picked Walz as her running mate.
"We love Joe. Joe has been an incredible president, but he just isn't the same messenger. And sometimes you need a better messenger," said Dan Miller, from Pelican Lake, Wisconsin, who attended the Walz-Harris rally. "And that's Kamala."
The momentum could be pivotal in Detroit, which is nearly 80% Black, where leaders for months had warned administration officials that voter apathy could cost them in a city that's typically a stronghold for their party. Rev. Wendell Anthony, president of the NAACP Detroit branch, said the excitement in the city now is "mind-blowing." He likened it to Barack Obama's first run for president in 2008, when voters waited in long lines to help elect the nation's first Black president.
Some Democratic leaders in Michigan had grown concerned that choosing the wrong running mate could slow that momentum, however, and fracture a coalition that has only recently started to unify.
Arab American leaders, who hold significant influence in Michigan due to a large presence in metro Detroit, had been vocal in their opposition to Shapiro due to his past comments regarding the Israel-Hamas conflict.
Those leaders specifically pointed to a comment he made earlier this year regarding protests on university campuses, which they felt unfairly compared the actions of student protesters to those of white supremacists. Shapiro, who is Jewish, has criticized Israeli Prime Minister Benjamin Netanyahu while remaining a staunch supporter of Israel.
Osama Siblani, the publisher of the Dearborn-based Arab American News and a prominent leader in Michigan's large Muslim community, was among those who met with White House adviser Tom Perez in Michigan last week. Although Perez was in the state on official business, he has maintained contact with some Dearborn leaders since he and other top officials traveled there with Biden in an effort to mend ties with the community.
Siblani said he met with Perez for over an hour on July 29 and told him that if Harris chose Shapiro, it would "shut down" future conversations.
"Not picking Shapiro is a very good step. It cracks the door open a little more for us," said Siblani, who along with Hammoud emphasized that any meaningful conversations must include policy discussions.
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Joey Cappelletti reported from Michigan. Associated Press writers Mark Vancleave, in Eau Claire, Wisconsin, Scott Bauer in Madison, Wisconsin, Tom Krisher in Detroit, Isabella Volmert in Lansing, Michigan contributed to this report.