The Hillary Clinton campaign is sounding the alarm over rival Donald Trump’s sudden fundraising surge, alerting donors in an internal memo that his $80 million July haul was “far more” than expected and should serve as a “wake up call.”

“We must redouble our efforts in the coming weeks,” campaign manager Robby Mook said in the memo, obtained by Fox News.

The memo was circulated after Trump and the GOP last week reported raising $80 million in July to Clinton and the Democrats’ $90 million. That marks a huge surge in fundraising for the Republican candidate whose monthly haul, up until July, amounted to a fraction of that.

Though Trump has slipped in the polls since the two parties held their national conventions, Mook noted in the same memo that the race remains “fluid” and predicted the polls will “tighten” again.

“[W]e have to take seriously the threat that Donald Trump could outraise us,” Mook said, in a memo first reported by Politico.

Presidential campaigns routinely alert donors to the possibility of being outraised, in a bid to drum up donations throughout the cycle, but the latest memo included some pointed warnings. Mook said Trump “only recently started earnestly fundraising from small donors and has clearly seen significant growth in the past month.”

He said they expect that support to keep growing in the final three months, adding: “Trump also has the capacity to write his campaign an eight or nine figure check, which could single-handedly tilt things in his favor.”

The warning from the Clinton camp comes as Trump tries to reset his own campaign, returning to talk about policy and his Democratic opponent on the campaign trail – after getting caught up in post-convention controversies seen as distracting from his core message.

Speaking Tuesday with Fox Business Network, Trump voiced confidence about his sustained appeal.

“I'm getting massive crowds,” Trump said. “… It's got to indicate something good. Every arena is sold out. We're having 15, 18 and 20,000 people, that's not even big. So you know something's going on out there. We'll find out on November 8.”

Trump delivered an economic address Monday in Detroit where he called for cutting taxes and regulation, and overhauling America’s international trade pacts, as the way to “jumpstart” the job market.

At the same time, he continues to deal with critics inside his own party. In the latest example, a group of 50 former security and foreign policy officials from GOP administrations dating back to Richard Nixon's penned a letter opposing Trump’s candidacy.

“None of us will vote for Donald Trump,” they wrote. “… Most fundamentally, Mr. Trump lacks the character, values, and experience to be President.”

Trump, in response, called them the Washington “insiders” and “elite” that he’s running against.

“The names on this letter are the ones the American people should look to for answers on why the world is a mess, and we thank them for coming forward so everyone in the country knows who deserves the blame for making the world such a dangerous place,” he said in a written statement.

Fox News’ Mike Emanuel contributed to this report.