WASHINGTON (Reuters) - Democratic presidential candidate Hillary Clinton said on Sunday the politically damaging "drip, drip, drip" of revelations about her use of a private email server is out of her control and she is unsure when the controversy might end.

Clinton, who has seen her lead shrivel in the race for the 2016 Democratic presidential nomination, said she has tried to be as open as possible and take responsibility for the email flap.

"It is like a drip, drip, drip. That's why I said there is only so much I can control," Clinton told NBC's "Meet the Press."

But asked if she could reassure nervous Democrats that no new email revelations would hit her campaign, she said: "I can't predict to you what the Republicans will come up with, what sort of charges and claims they might make."

Clinton compared criticism about her use of private email instead of a government account while she was secretary of state to the flood of controversies and Republican-led investigations that marked the presidency of her husband Bill Clinton in the 1990s.

"I have been involved from the receiving side in a lot of these accusations," Clinton said. "In fact as you might remember during the 90s there were a bunch of them. All of them turned out to be not true."

Clinton has apologized for the email set-up and said it was a mistake. She gave 55,000 pages of work-related emails to the State Department last year but eliminated about 30,000 emails she said were personal. On Sunday, she said she did not help her lawyers determine which ones to turn over.

"I did not want to be looking over their shoulder," she said, calling accusations she was trying to avoid transparency laws "ridiculous".

A new Wall Street Journal/NBC News poll on Sunday found Clinton's lead over top rival Bernie Sanders, a U.S. senator from Vermont, has dwindled to 7 percentage points, 42 percent to 35 percent, amid the controversy.

Asked about the hit in polls, Clinton said "what I have tried to do in explaining this is provide more transparency and more information than anybody I'm aware of who has ever served in government."

The most recent revelation was a report on Friday about an email exchange with former CIA Director, retired Gen. David Petraeus, that she did not turn over, and which occurred before she said she had set up her personal account.

Clinton said the private server was already in her house because her husband had set it up after leaving office, and she just added her account to it.

"What we had available at the time was turned over," she said. "I wasn't that focused on my email server."