By Steve Holland
MANCHESTER, N.H. (Reuters) - Democratic presidential candidate Hillary Clinton's campaign was hit on Friday by the FBI's reopening of its investigation into her use of a private email server while secretary of state, eroding a political boost from a strong U.S. economic report.
With just 11 days to go before the Nov. 8 election, FBI Director James Comey said in a letter to several congressional Republicans that the agency had learned of the existence of emails that appeared to be pertinent to its investigation.
However, he said the FBI did not know if the emails were significant and did not provide a time frame for the probe.
Republican Donald Trump's campaign reacted with glee. His campaign manager, Kellyanne Conway, said on Twitter that "a great day in our campaign just got even better."
The resurrection of the email issue, which has dogged Clinton's campaign from the start, dimmed a day that had featured good news for her effort to win the White House.
The Commerce Department reported that the economy grew at a 2.9 percent annual rate in the third quarter, its fastest pace in two years and higher than the expected 2.6 percent, thanks to a surge in exports and a rebound in investment.
The report had bolstered Clinton, who has positioned herself as the best candidate to continue years of economic expansion under Democratic President Barack Obama.
More Americans say jobs and the economy are their No. 1 priority when they decide who to vote for than any other issue.
Trump argues that as a successful businessman and political outsider, he is the best person to take a new approach to rebuilding an economy that has sent too many jobs overseas and left many Americans struggling to find decent jobs.
His campaign said the figures are still not good enough.
"America can do better than the modest growth of 2.9 percent recorded for the 3rd quarter and the dismal growth of 1.5 percent for the past year," Dan Kowalski, Trump's deputy policy director, said in a statement.
While many voters do not follow economic indicators closely, outside experts said the release was still a good one for Clinton. She is seeking to solidify her lead in opinion polls as the Democratic Party works to win as many seats as possible in the U.S. Senate and House of Representatives, where Republicans now control majorities.
Clinton has also been looking to broaden the electoral map. Her campaign said on Friday that she would campaign in Arizona next week.
"Today's release will likely improve the perception of economic conditions in the U.S. and slightly increase the odds of a Democratic president remaining in the White House," said Brian Schaitkin, senior economist at the Conference Board.
Clinton's camp said Friday's report showed "real progress" since Obama took office in 2009, when the country was struggling to emerge from economic recession.
"With more than 15 million jobs created since early 2010 and real median incomes growing more than 5 percent last year, it's clear we've made real progress coming back from the crisis," Clinton senior policy advisor Jacob Leibenluft said in a statement.
But he added that there is still more that can be done.
Clinton was campaigning on Friday in Iowa, where polls show she and Trump running neck-and-neck, and in Michigan, a traditionally Democratic state hit hard by the movement offshore of many formerly well-paying American manufacturing jobs.
Trump was holding rallies in Iowa as well as in another closely contested swing state, New Hampshire, and in Maine, where his campaign sees a chance to grab one of four electoral votes.
(Additional reporting by Doina Chiacu and Patricia Zengerle in Washington; Writing by Patricia Zengerle; Editing by Alistair Bell)