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Hillary Clinton declared her candidacy for the White House on 12 April 2015. That means she has had 514 days to figure out a good response to the email controversy.

As she demonstrated at the candidates' debate last night, so far she's failed.

That failure doesn't disqualify her from being president, but it does say something interesting about her.

Mrs Clinton's supporters are frustrated that this issue is dominating her campaign to the extent that it took up a third of her time at a forum ostensibly about national security.

Why is this such a big problem for her?

Is it the substance of the issue - the fact that she had a private email server and used it to send work-related emails while she was secretary of state - or is it how she's handled it subsequently?

I'm inclined to think the latter.

After all, the email story is very complicated.

Most people, myself included, don't really understand all the technical details of it.

Also, many of the emails from that address have still not been released, so it is hard to judge the substance and, of the ones that have, there is so far none which appears to compromise US national security.

The more damaging thing is how she's handled it.

As one Democratic senator, and an ardent supporter of Mrs Clinton, put it to me this week, "the problems are all of her making".

What infuriates Clinton fans and friends is that she hasn't managed to put the issue behind her with a simple, heartfelt apology.

They wish she would come up with a contrite phrase, something like "it was a big mistake, I really wish I'd never done it. I am very sorry. I take the security of our country extremely seriously and have never compromised it".

Then they say, she should repeat that phrase again and again and again until we are all so tired of it that the issue dies from collective boredom.

After last night's grilling that seems like sensible advice.

For whatever complicated reason, however, it is something she finds impossible to do.