COLUMBUS, Ohio — Hillary Clinton’s supporters here are nervous about her campaign, fearing the renewed scandal over her emails will dent her — and she is hoping to counter with a full slate of surrogates urging voters to back the Democratic team, even if they are not excited about its star player.

“I think it is making people doubt her again,” said Connie Boehner, who attended a Clinton rally this week. “Every time I think we get past all this nonsense, much of which has not come close to being proven, this stirs people up.”

A Washington Post/ABC tracking poll released Tuesday showed Mrs. Clinton and Republican presidential nominee Donald Trump running neck and neck in the race for the White House, and that enthusiasm for the Democrat has dissipated after last week’s announcement by the FBI that it found a tranche of messages tied to Huma Abedin, Mrs. Clinton’s closest personal aide.

Mrs. Clinton and her team say there is nothing new in the investigation, but saturation coverage of Mr. Comey’s decision has shaken the race and left her supporters worried.

“I know a lot of people there that were on the fence are sort of swinging back to Trump because of the email stuff,” Ms. Boehner said.

President Obama looked to tamp down those concerns Tuesday at a rally on the campus of Capital University, where he told voters Mr. Trump is a con artist who can’t be elected.

“Come on, this guy?” Mr. Obama said. “Don’t be bamboozled.”

The president, who battled Mrs. Clinton in the 2008 Democratic primary race before choosing her as his secretary of state, insisted she is ready to lead and promised she will continue his agenda.

But he was aware of the danger that her supporters could stay home.

“Make no mistake, this is not something you can take for granted. All the progress we made goes out the window if we don’t do our jobs in these next seven days. Our future depends on what you do these next seven days,” he said.

Mrs. Clinton could use some of the magic from Mr. Obama’s Ohio campaigns, where he rode a wave of optimism and his “change” slogan to a 4-percentage point victory over Sen. John McCain in 2008, and a 3-point victory over Mitt Romney in 2012.

The Real Clear Politics average of polls shows the former first lady trailing Mr. Trump in Ohio by an average of 2.5 percentage points, and voters are open about how their enthusiasm is lagging from where it was four years ago.

“I was more confident in Barack Obama,” said Daniel Shephard, 60, of Columbus, adding that he thinks Mrs. Clinton is the lesser of two evils in this election.

The Clinton reinforcements will continue Wednesday when Anne Holton, the wife of Mrs. Clinton’s running mate, Tim Kaine, parachutes into Ohio for a series of campaign events and Thursday when Sen. Bernard Sanders is scheduled to make a couple of stops on her behalf.

Mrs. Clinton also plans to return to the state Friday for a rally in Cleveland with rapper Jay Z.

Joey Gutter of Columbus said the problem for Mrs. Clinton is that the email investigation plays into the politics of fear on which Mr. Trump has based his campaign.

“He is just trying to poke and pry at fear and the unknown, and I definitely think it hurts here,” the 27-year-old Clinton supporter said. “I think that people who don’t think of it like I do, think it is an unknown and they don’t trust her because of it.”

Others said they suspect the impact of the revived email controversy will be a wash.

“I think it will bring out the vote on both sides — hopefully it will just bring out more for her,” said Betsey Krause, 57. “She absolutely is going to win this thing if people use their brains.”

Ohio has been a good barometer in presidential races, having voted for every winner in since 1964. The last time it lined up behind the loser was when it supported Republican Richard Nixon over Democrat John F. Kennedy in 1960.

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