Walz was a hit in the Midwest. He’s a ‘question mark’ in the Sun Belt.

Tim Walz helped Kamala Harris fire up the base on his Midwestern home turf.

Now, Democrats are about to find out how he plays outside that region. The new running mates are turning their attention from the Rust Belt to the Sun Belt — testing whether Walz’s folksy, Midwestern brand can resonate beyond middle America.

Harris’ campaign is betting that Walz’s biography — he is a former schoolteacher and football coach who grew up in small-town Nebraska, served in the Army National Guard and held a battleground House seat in Minnesota for...