Tim Walz has power with people from his past ... 'cause he's changed the mind of a former player he mentored decades ago when he was a Minnesota high school athletics coach.

Walz -- the current Minnesota governor and Kamala Harris' VP choice -- was the football coach at Mankato West High School in Mankato, MN, for several seasons in the '90s -- and he took a student named Dan Clement under his wing.

TMZ caught up with Dan on Wednesday and asked about his experiences with Walz as a teenager. Dan told us Walz had such an impact on his life that he went from an undecided vote to the Harris/Walz Democratic ticket in the upcoming presidential election.

Of course, Donald Trump -- the Republican nominee -- is no longer under Dan's consideration.

Dan says Walz was good to him when he was a troubled adolescent by being attentive and helping him get on the right track instead of brushing him aside.

Dan also noted every player on his team mattered to Walz ... who showed them all true compassion, which he believes Americans will experience if he and Kamala win the White House.

As we've reported, Walz has been flooded with glowing reviews by people from his past, including many of his ex-students, since his VP candidacy was announced this week.

One former student from his geography class hailed him as the ultimate role model ... revealing how he fought for inclusivity while sticking up for his gay students against homophobic bullies.