US election 2016: How damaging is this for Clinton?

It could be nothing. It could be everything. And it almost certainly won't be resolved before Americans head to the polls in just under two weeks.

The letter from FBI head James Comey to Congress explaining that his agency is once again assessing Hillary Clinton's emails for possibly mishandling classified information is frustratingly vague.

There are no details, for instance, or how many emails are in question.

That will only fuel the rampant speculation breaking out, with leaks from "government sources" already fanning the flames.

The new batch of emails are said to come from an investigation into allegations that former Congressman Anthony Weiner, estranged husband of Clinton confidant Huma Abedin, exchanged sex-related messages with an underage girl.

The FBI is apparently pushing back on the characterisation that this is a "re-opening" of its Clinton investigation, telling CBS that such conclusions are "premature".

Premature conclusions are Washington's stock in trade, however.

Mrs Clinton's critics are going on the attack, using the latest news to support their claims that the former secretary of state engaged in malfeasance.

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Her presidential opponent, Donald Trump, wasted no time branding her as a corrupt criminal during his afternoon rally in New Hampshire.

As the crowd chanted "lock her up", he asserted that the FBI may now have a chance to show the "courage to right the horrible mistake that they made" in previously declining to indict the former secretary of state.

Meanwhile, Mrs Clinton's supporters will spend the next few days in a defensive crouch, trying to assess how bad the damage could be.

John Podesta, a senior advisor to the Clinton campaign, is already attempting to turn the focus back on the FBI and the reasoning behind the bomb it seemingly casually set off in the last days of this presidential campaign.

"[Mr Comey] owes it to the American people to immediately provide the full details of what he is now examining," he said in a statement. "We are confident this will not produce any conclusions different from the one the FBI reached in July."

Mrs Clinton herself held a rally in Iowa on Friday afternoon and made no mention of the unfolding drama. She later echoed Podesta's call for the FBI to release more information.

It seems unlikely that this will move many voters from Mrs Clinton to Mr Trump on election day - the partisan divide between the two camps at this point is too great.

It could depress the Democrat's turnout, however, or cause her to bleed support to third-party candidates like the Green Party's Jill Stein or Libertarian Gary Johnson.

What's certain is that whether this ends up being a big deal or not, it places the spotlight on all the wrong places for the Clinton campaign.

It will drown out her closing message to voters and dominate the political conversation for days.

Perhaps most importantly, it all but guarantees that even if she wins the White House, the early days of her presidency will be dogged by this long-running political imbroglio.

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