In a presidential election that will be decided by the slimmest of margins, every decision counts.

But has Kamala Harris made a mistake with her choice of running mate? 

A new DailyMail.com/J.L. Partners poll shows that she is tied with Donald Trump in the likely tipping point state of Pennsylvania. 

Yet she would be ahead if she had added its governor, Josh Shapiro, to her ticket instead of Tim Walz from Minnesota. 

When likely voters were asked who they would back, 47 percent said Harris/Shapiro, one point more than said they would vote for Donald Trump and J.D. Vance.

But when it was Walz on the ticket, the rivals were tied 47 points apiece.

The numbers bear out what Democrats and Republicans alike said at the time: Harris may have missed a trick by passing over the popular governor of a swing state that could decide who wins the election. 

David Urban, Trump confidant and longtime Pennsylvania strategist said: ‘Josh Shapiro is an incredibly talented, gifted political athlete.

‘And I think it just speaks to Kamala Harris’s poor judgment and the inherent antisemitism and anti-Zionism in the Democratic Party that she didn’t pick such a clear winner.’

Shapiro is Jewish and has been governor since 2023.

Polls regularly label him the state’s most popular politician—more popular even than Taylor Swift, who spent her early life in West Reading, Pennsylvania.

However, when Harris was making her decision, some progressives highlighted his stance on Israel and his criticism of pro-Palestinian protesters.

He may not have won a place on the ticket, but Harris made a nod to the importance of his state, which represents 19 electoral college votes, when she unveiled her choice. She introduced Walz, the governor of Minnesota, as her running mate at a boisterous rally in Philadelphia.

Running mates rarely decide the outcome of the election. 

But James Johnson, co-founder of J.L. Partners, said with such a tight race Harris may have weakened her chances of victory.

‘Walz has not been a disaster by any means—even in the debate he technically lost to J.D. Vance, he didn’t put voters off. 

‘But it is hard to deny that picking a hugely popular governor of the most important state might have been the savvier decision. 

‘This road is laced with counterfactuals—not least whether Shapiro could have performed as well under the national spotlight. But for now it looks like he would have been a better pick than Tim Walz.’

Polls suggested that Walz came off second best in his debate against Vance.

And he has been forced to repeatedly clarify comments on everything from his whereabouts during the 1989 Tiananmen Square massacre to his position on the Electoral College.

For now, the election is on a knife-edge. The latest data from the J.L. Partners/DailyMail.com election model show that Trump has the lead, with a 58 percent chance of victory.

But that means the race will go down to the wire.