Top Democrats are warning that Hillary Clinton could still lose the presidential election in November, according to a report Wednesday in Politico.

Over the last two weeks, they've communicated that message on conference calls and in private conversations, the report said, and some Democrats on and off Capitol Hill have talked about it publicly.

Senate Minority Leader Harry Reid, D-Nevada, for example, who's retiring from Congress, recently expressed that concern.

"We can't be over-confident," he said after campaigning for Clinton in Las Vegas, adding that he has experienced first-hand having high expectations that didn't pan out. "I really am [worried about that]. I've been there, done that."

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A deputy secretary of state who served under Clinton, Tom Nides, told Politico that her campaign has to treat this like it's an incredibly close race between the two major party nominees.

"Although obviously you'd rather have these numbers than not, these poll numbers are completely inflated, and I think the campaign has got to operate as if it were a dead-even race," Nides said, according to the report, adding that campaign officials seem to understand the stakes.

"They're not letting up, they're trying to make sure people understand it's going to be a very close election," he said.

On a private weekly call with Clinton's finance council last week, her spokesman Brian Fallon told supporters that the campaign expects national polling to eventually tighten. Clinton's campaign manager, Robby Mook, also sent a note called "Wake Up Call" to top fundraisers, the report said, warning that Trump could out-raise Clinton before November.

"None of us wants to wake up on November 9th wondering what more we could have done to prevent a Trump presidency," Mook wrote, according to the report. "The threat to our country and the world is too great to leave any room for second-guessing ourselves. With Donald Trump and his supporters showing that they are raising the resources they need to compete, we need to do the same."