In short, when she ran for Senate against Rick Lazio, a nobody, she only won 55 to 43 (boldface mine):

Essentially, no matter how good a president she might or might not be, Hillary Clinton isn’t the greatest candidate. When she goes on the campaign trail and faces competition, she tends to do worse for it. There are a lot of theories about why that is, but my personal take on it is that it’s about demeanor. Hillary Clinton has a very reserved campaign demeanor. She rarely gets visibly upset, and she almost never loses message discipline or does anything that one might consider a campaign “stunt.” It’s all steady as she goes.

The problem with that is that it makes her look robotic to many eyes. At a time when people are hurting and respect for elites is at an all-time low, voters are looking for someone who will channel their anger and resentment into real change. Republican voters are getting that with Donald Trump, and liberal Democrats are getting it with Bernie Sanders.

Clinton’s email problems are certainly hurting her, but it’s the aura of the candidate and the campaign that are more to blame for her polling declines. It’s not enough to say you will fight for the American people. You have to demonstrate it in your body language, your tone, and your choice of words. And sometimes you have to show that you’re willing to mix it up and do the unexpected and the controversial.

The Clinton approach does none of those things, and it’s showing in the numbers.

I certainly don’t think this is the only thing that’s lowered her poll numbers, but I’ve never seen such mirthlessness and joylessness on the campaign trail. And the ‘women can’t be passionate without seeming screechy’ excuse doesn’t work: plenty of women, on the left and right, can and have run effusive and inspiring campaigns.

Leaving aside serious policy concerns, watching Clinton feels like doing homework or cleaning the bathroom. Not fun. At all.

For people who already think politics is a chore, not an interesting hobby, this doesn’t help.