If Donald Trump wins, it will be because Kamala Harris made the wrong decision about her running mate. With the current electoral map – where Arizona, Nevada, Georgia, North Carolina, Pennsylvania, Michigan and Wisconsin are considered the swing states – Democrats have 20 different possible winning combinations, while Republicans have 21.

Were Republicans to win Pennsylvania, Democrats still have 10 other ways to win the race; if Democrats carry the Keystone State, Republicans have just six. If Nevada and Arizona end up in Republicans’ hands, as some recent polls indicate, Pennsylvania becomes even more critical to Democrats: they have only one remaining route to the presidency without it, but six with it. For both parties, Pennsylvania is a crucial state, but if Nevada and Arizona are trending more Republican, then it becomes absolutely vital to Democrats.

Harris missed a vital opportunity to make her road to victory much easier when she chose Minnesota Gov. Tim Walz as her running mate over Pennsylvania Gov. Josh Shapiro. It’s not just that Shapiro is governor of the state, either. He’s the most popular politician in the state, and more popular and better known nationally than Walz. Now, putting Shapiro on the ticket wouldn’t have made winning the election, or even his home state of Pennsylvania, a slam-dunk for Harris – just easier.

In modern presidential politics, running mates aren’t typically chosen for geographical reasons any more. If they were, Harris, Biden, Pence and Cheney all never would have been chosen. Still, one doesn’t often have the chance to pick a popular running mate from such a vitally important state, either. Trump didn’t even have anyone like that on his list this year. Even if he’d wanted to choose someone for that reason, he’d have had a tough time finding somebody; there’s simply no Republican figure comparable to Shapiro in a competitive state.

It’s also hard to find any other one decision or moment that could have affected the race in such a clear, specific way with one simple decision. No other events – their debate, the assassination attempts against Trump, hurricanes, the war in Gaza, other endorsements – seemed to have a notable effect on the polls. While Harris got a bounce out of being rolled out as the nominee, she’s settled back to earth since then, with the race gradually shifting back in Donald Trump’s favor.

As to why Harris didn’t select Shapiro as her running mate, a number of explanations have been offered, but none of them quite make sense. One is Democratic divisions over the war in Gaza. Shapiro has been outspoken in favor of Israel on the issue. While that might have dissuaded Arab-American voters – especially in Michigan – from voting for the ticket, they don’t really have anywhere else to go. Trump has made it clear that he’s adamantly pro-Israel, so even if they simply didn’t vote, they risk helping to put him back in the White House. It might have taken some time, but it’s hard to imagine they wouldn’t have been brought back around eventually. Another is that they just didn’t click personally, but that’s often the case between presidential candidates and their running mates; it’s not a good reason in and of itself.

One can, perhaps, point to other, more abstract decisions she and her campaign team have made, like her strategy of generally avoiding mainstream media interviews early on, but it’s hard to know the effects of that. Sure, maybe giving more interviews would have been a better way to introduce her to the American people, but one disastrous outing could have destroyed her candidacy – and then she recently abandoned that strategy and decided to do a Fox News interview. Similarly, we’ll never know whether a more aggressive strategy to differentiate herself from President Biden would have worked, or simply divided the party even further.

Now, we can never prove any counter-factual, so we’ll also never know for sure that choosing Shapiro would have assured her of a victory in Pennsylvania or as a whole. It’s simply that it’s one particular decision that would have undoubtedly had a major impact on the race and was entirely under the control of the candidate themselves, just like Trump’s decision to go along with the request for an earlier presidential debate.

There are always a whole host of factors that go into any candidate winning a presidential election, but this year we have two clear examples where both candidates could have made different decisions that would have served them better, and that’s a rarity.