Why is Harris doing well? Simple: Trump.

Republican presidential nominee Donald Trump erred greatly in selecting Sen. JD Vance of Ohio as his running mate. Trump’s overconfidence in the race against President Joe Biden led to him prioritizing the MAGA legacy.
Trump’s misstep became clear once Biden dropped out and endorsed Vice President Kamala Harris. The presidential election quickly shifted.
Harris then picked Minnesota Gov. Tim Walz, and that campaign seems to be continuing its positive momentum, even if Republicans have widely criticized the Democratic vice presidential nominee.
The election changed, the polls changed and Trump was left with Vance. But the choice of Vance is just a symptom of an issue that has plagued Trump for years: his overconfidence in his own handpicked candidates, and Republican willingness to just play along.
Now, as a result of Trump’s cockiness, he has jeopardized both the election and his legacy ‒ and handed Democrats the momentum they’re enjoying as the Democratic National Convention begins Monday. Trump continues to be his own worst enemy.
A month ago, the race appeared practically over. Biden was reeling from an awful debate performance and all-time low approval ratings. The polling was horrible for him.
On the other side, America had just watched Trump survive an assassination attempt, raising a fist in the air to his supporters after being hit in the ear days before the Republican National Convention. Trump was on the verge of a blowout.
Then, in a matter of weeks, Trump chose Vance, Biden stepped down from the reelection race, and Harris took control of the Democratic ticket and, seemingly, the election.
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Harris essentially gained all the ground that Biden had lost to Trump. Despite polling generally even with Trump, the sentiment was that she had an uphill battle to regain the trust that Americans had lost in Biden. 
Since then, however, Harris has steadily climbed in the polls, inching ahead of Trump gradually but noticeably. 
To put this massive swing in momentum into perspective, on July 15, Trump had an implied 66.2% chance of winning the November election. Since then, his chance has fallen by more than 20 percentage points, solidly trailing Harris. 
Had you asked me a month ago what polling would look like right now, I would have forecast that Trump was running away with it. Even if I had known for a fact Harris would be stepping in, I likely wouldn’t have changed my opinion much. 
So this massive momentum shift leads me to one question: What are the Democrats doing so right?
In my view, almost nothing. Harris’ steady rise in polls can be chalked up to simply her not making any particularly offensive decisions and the dissatisfaction with Biden among Democrats.
But the biggest thing helping Democrats is that Trump is electoral poison.
Outside of his surprise victory in 2016, Trump has actively harmed the GOP electorally ever since he entered public office.
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In the 2018 midterm elections, Democrats clawed back the House of Representatives. 
In 2020, when Trump lost the presidency, the Republican Party lost the Senate and failed to regain control of the House. Worse yet for Trump, Democratic House candidates ran behind Biden nationally, offering more indications that the top of the ticket was the problem. 
The 2022 midterm elections only confirmed that hypothesis, as what was expected to be a huge red wave for Republicans turned out to only be a trickle. While Republicans regained control of the House, they failed to regain control of the Senate, largely due to Trump’s failed endorsements across the nation. 
Now, in 2024, Trump’s overconfidence and narcissism could once again cost Republicans electorally. In choosing Vance and successfully coalescing the Republican National Committee behind himself as the party platform, Trump did away with the widespread appeal Republicans could have enjoyed on the backs of four awful years of Biden. 
Rather than the possibility of Republican unity and growing electoral influence, Trump has chosen to further sow divisions in his own party, fully converting the GOP into the party of Trump. Rather than build bridges with the factions of the GOP skeptical of Trump, he has instead chosen to push our movement further from the roots that he built his 2016 campaign upon.
Trump and Vance have such poor electoral appeal that the extremely liberal Walz has done little to help Republicans. While Harris has her own issues as a presidential nominee, they are nothing compared with the baggage Trump brings to the election.
Such a tumultuous campaign only opens the door for more electoral struggles. Trump could still very well win, but a loss in 2024 would be horrible for his legacy, as it would be the fourth election in a row that Trump has actively harmed his own party’s election success. 
In reality, the best thing for Democrats is GOP incompetence and Trump’s own ego. This has been true the whole time.
Dace Potas is an opinion columnist for USA TODAY and a graduate of DePaul University with a degree in political science.