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While FBI Director James Comey’s email bombshell may not dent Hillary Clinton's White House chances, Democrats fear it might shorten her coattails and threaten their prospects of retaking the Senate.

Comey’s decision to publicly announce a new front in the Clinton private email server investigation so close to the election has come under fire from Democrats and even some Republicans.

Its impact on the presidential race may be marginal, since Clinton has built up a comfortable cushion of electoral votes and Donald Trump pools poorly with some key demographic groups.

But small disruptions in tight down-ballot races could have a big impact and the Democrats’ aggressive response to Comey belies their concern that his move could cost them at the polls.

“It's certainly not helpful,” said Missouri Democratic Party Chairman Roy Temple. “It kind of pollutes the Democratic brand in a way that’s unnecessary, simply because it doesn't actually involve any new information, which is why the frustration at Comey is so high right now."

Missouri is one of at least six states with neck-and-neck Senate races, each of which could theoretically tip the balance in Congress' upper chamber. So even small changes in turnout could have outsize impact.

"The slightest breeze in any direction can really push these races one way or another,” said Ian Prior of the Senate Leadership Fund, a deep-pocketed GOP Super PAC. "All you need is need is .01 (percent) in a race like this."

Related: Democrats Take Aim at GOP Senate Majority

Prior’s group is using the FBI investigation to boost get-out-the-vote efforts in Nevada and Missouri with digital ads targeting base Republican voters that highlight the latest Clinton "scandal."

"There's no question that Clinton has a better ground operation," said Republican strategist Brian Walsh. "But there are few things that energize Republicans more than a Clinton scandal."

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Democrats are closely watching early vote returns for any drop-off in Democratic turnout, or increase in Republican enthusiasm, but have so far been relieved to see their worst fears unrealized.

"Our biggest passion is about the overall turnout, and that's why I think those down-ticket candidates are buoyed by what a good day we saw on Saturday," said Ohio Democratic Party Chairman David Pepper, touting record-breaking showing in major cities on the first day of early voting after the Comey news broke.

In North Carolina, where Democrat Deborah Ross is challenging Sen. Richard Burr in tight race, Democrats there likely breathed a sigh of relief when turnout on Saturday also surpassed 2012 levels.

Related: DOJ to Lawmakers: Working 'Expeditiously' With FBI on Emails

Still, some say that they think the FBI news has foreclosed the possibility of the kind of election triumph that seemed within reach just a few weeks ago.

“It moves us closer to the middle of the bell curve in terms of possible outcomes,” said one Democrat working on Senate races, granted anonymity to speak candidly, and who noted that polls had been tightening anyway before the news.

Patrick Murray, the director of the independent Monmouth poll, agreed. “While the email news does not play a decisive role in the presidential contest, a couple of points on the margins could be having a critical impact on tight down-ballot races,” he said.

Related: FBI Director Comey Opposed Naming Russia in Hacking Probe: Officials

Murray new survey of Indiana’s tied Senate race found only 4 percent of voters said the FBI news had changed their minds, but that’s easily enough to swing the outcome, if those voters follow through on their stated preference.

Republicans have tried to use Comey’s letter to Congress Friday to drive a wedge between Clinton and down-ballot Democratic candidates, eager to turn the tables after GOP candidates spent months in awkward in an dance with their party’s nominee.

Related: Poll: Clinton Maintains National Lead Over Trump Despite FBI Letter

“Democrats should have to answer if they will continue to defend their nominee as she continues to jeopardize our national security,” said Andrea Bozek, communications director for the National Republican Senatorial Committee.

But Democrats up and down the ballot are confident there will be zero defections.

“The bizarre and concerning action taken by FBI Director James Comey will not cause Democratic voters or candidates to waver in their support for Secretary Clinton,” said Democratic Congressional Campaign Committee spokesperson Meredith Kelly. “This continues to be a no-brainer.”

In fact, Democrats argue the news has only energized their base.

“A lot of our grassroots folks are just angry at what the director did and it's really motivated them to work even harder,” said New Hampshire Democratic party Chair Ray Buckley, summarizing the sentiment he heard at the party’s recently renamed Kennedy-Clinton fundraising dinner, held Saturday. “It could have gone either way, but it's certainly been a boost to activity.”

And they say it could remind their supporters that President Trump is a possibility if they don’t get to the polls. "Our fears of complacency have now disappeared,” Buckley said.

Still, that’s the party faithful. Democrats acknowledge the impact could be different on the few remaining undecided voters, as well potentially drive Trump-skeptical Republicans to the polls.

A new NBC News Survey Monkey poll showed that among independent voters who do not lean towards either party, 68 percent said Comey’s actions were an important issue for the campaign, while only 31 percent said it was a distraction.

Republicans think the Comey news will boost their case for control of Congress to provide a “check-and-balance” on Clinton — an idea they think could appeal to even some soft-Clinton voters.

“The big thing with this is not necessarily the substance of what the FBI is or isn't looking at,” said Prior, of the Senate Leadership Fund. “It's the final nail in the coffin where people realize this kind of thing is never going to stop with Hillary Clinton. And maybe it's too late to stop her from becoming president, but it's not too late to make sure that she's checked by a Republican Senate."

Democrats are still hopeful that Comey could undo much of the damage he’s caused by quickly announcing the new emails have no relevance.

The best-case scenario for Democrats is likely to be banking as many of their loyal early voters as they can and hoping for a better political environment by next Tuesday.