Tim Walz’s personal story spotlights IVF. That could be a problem for Republicans.

When Alabama’s high court earlier this year ruled that frozen embryos are people, temporarily halting in vitro fertilization treatments at some clinics, Democrats turned it into political gold.

Tim Walz gives them an opportunity to do it again.

At a Tuesday rally in Philadelphia, the Minnesota governor for the first time on a national stage shared how he and his wife, Gwen, struggled to conceive. And by Wednesday, he had cemented the story as part of his stump speech.

“This is very personal for my wife and I,” Walz told...