Arizona Sen. Mark Kelly, who made the veepstakes short list, wrote on X that Vice President Kamala Harris and Gov. Tim Walz "are going to move us forward."

"They’re already building a campaign to unite our country -- and [Gabby Giffords] and I are ready to do everything we can to help them win."

-ABC News' Tommy Barone

Gov. Tim Walz posted on X on Tuesday, saying he's "all in" on his new role as Harris' VP pick.

"It is the honor of a lifetime to join [Kamala Harris] in this campaign. I’m all in. Vice President Harris is showing us the politics of what's possible. It reminds me a bit of the first day of school. So, let's get this done, folks! Join us," he wrote, also linking to a donation page.

-ABC News' Brittany L. Shepherd and Fritz Farrow

Pennsylvania Gov. Josh Shapiro posted on X about Vice President Harris' choice of Tim Walz as a running mate, saying it was "a deeply personal decision for the Vice President -- and a deeply personal decision for me."

"Pennsylvanians elected me to a four-year term as their Governor, and my work here is far from finished -- there is a lot more stuff I want to get done for the good people of this Commonwealth," he wrote, in part, in a statement.

"Over the next 90 days, I look forward to traveling all across the Commonwealth to unite Pennsylvanians behind my friends Kamala Harris and Tim Walz and defeat Donald Trump," he said. "See you tonight in Philly."

-ABC News' Will McDuffie

Vice President Kamala Harris took to Instagram to announce Minnesota Gov. Tim Walz as her vice presidential running mate. Her X page was also updated with a new social wrap that highlights her vice presidential pick, along with a new poster that features the two.

"I am proud to announce that I've asked [Gov. Tim Walz] to be my running mate," Harris wrote on Instagram.

"One of the things that stood out to me about Tim is how his convictions on fighting for middle class families run deep. It's personal," she continued.

"He grew up in a small town in Nebraska, spending summers working on his family's farm. His father died of cancer when he was 19, and his family relied on Social Security survivor benefit checks to make ends meet. At 17, he enlisted in the National Guard, serving for 24 years. He used his GI Bill benefits to go to college, and become a teacher. He served as both the football coach and the advisor of the Gay-Straight Alliance," Harris wrote.

"I share this background both because it’s impressive in its own right, and because you see in no uncertain terms how it informs his record. He worked with Republicans to pass infrastructure investments. He cut taxes for working families. He passed a law to provide paid family and medical leave to Minnesotan families," she continued. "He made Minnesota the first state in the country to pass a law providing constitutional abortion protections after the Supreme Court overturned Roe v. Wade, and as an avid hunter, he passed a bill requiring universal background checks for gun purchases."

"But what impressed me most about Tim is his deep commitment to his family: Gwen, Gus, and Hope. Doug and I look forward to working with him and Gwen to build an administration that reflects our shared values," she said.

-ABC News' Gabriella Abdul-Hakim, Fritz Farrow and Will McDuffie