At the Iowa State Fair last year, I ran into two sisters, Diana Barker and Peg Wills. Barker was a Democrat who observed Donald Trump sweeping through the crowd with a surprising receptivity, though she didn't like the things he'd said about Mexicans at his campaign launch. Wills was a Republican who was leaning toward John Kasich but hadn't made up her mind. "It makes so much sense to have a businessman be president," Barker said. Wills offered a counterpoint: "Trump going overseas to say what he thinks might not be the wisest choice."

That's Trump's problem in a nutshell. People find his business experience attractive but worry about his willingness to fly off the handle. Wills, as a Republican woman, is a member of the group that's most responsible for Trump's current polling problems. Democratic men prefer Hillary Clinton over Trump by 89 points, according to the most recent Post/ABC poll. Democratic women prefer her by 86 points. Republican men prefer Trump by 82 points — but Republican women prefer him by only 64 points.

Asked which candidate has a better personality and temperament to serve effectively as president, the gap is similar. Over 9 in 10 Democratic men and women say Clinton has the better temperament. But only 68 percent of Republican men pick Trump — as do only 58 percent of Republican women.

On Monday morning, Hillary Clinton unveiled a new ad that aims directly at this weakness.

An ad put out by Hillary Clinton's presidential campaign uses Republican rival Donald Trump’s own words to suggest that he would be dangerous as president. (Hillary Clinton)

The focus is on the president's responsibility to engage in military conflict, something of a mirror of the 2008 "3 a.m." ad Clinton deployed against President Obama. "All it takes is just one wrong move. Just one," it concludes, over the sound of a jet passing by.

Even among those who like Donald Trump, his temperament is a point of weakness. Only 3 percent of those who view Hillary Clinton favorably think she lacks the temperament to do the job, according to our most recent poll. By contrast, 17 percent of those who view Trump favorably think he lacks the proper disposition.

Asked to pick between Clinton and Trump on whose disposition is better, no current supporters of Clinton say Trump has got the better temperament. More than 1 in 8 Trump supporters, though, say Clinton's personality and temperament are preferable for the position.

What's more, 28 percent of Republicans overall say that Clinton's temperament is the better one. A majority of independents and a huge majority of Democrats say the same thing.

The target for this ad, in other words, is likely the same target Clinton (successfully) aimed at during the Democratic convention. She hopes to remind Republicans about their concerns over Trump's fitness for office and, at the very least, keep them from committing to their party's nominee.

She hopes, in other words, to keep Diana Barker's approval of Trump's business record from overwhelming Peg Wills's fears about what Donald Trump might actually do once he traveled overseas to confront our geopolitical foes.