Fresh off a big win in the West Virginia primary, Sen. Bernard Sanders‘ campaign said Wednesday the Democratic Party would be courting “disaster” if it nominates Hillary Clinton as its presidential nominee.

In a fundraising email to supporters, Sanders campaign manager Jeff Weaver cited recent polls showing the Vermont senator performing better against Republican Donald Trump in general election match-ups. Recent surveys have shown Mrs. Clinton virtually tied with Mr. Trump in the key battleground states of Ohio, Pennsylvania and Florida.

Citing those troubling figures, Mr. Weaver said the Democratic Party — and its superdelegates who are free to support either candidate — must reject Mrs. Clinton and embrace Mr. Sanders, or face a crushing defeat in November.

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“For months, Bernie Sanders has out-polled Hillary Clinton against Donald Trump, and often by extraordinarily large margins. Because we must do everything we can to defeat Trump in November, our mission is to win as many pledged delegates as we can between now and June 14,” when the primary season ends, Mr. Weaver said. “Then we’re going to have a contested convention where the Democratic Party must decide if they want the candidate with the momentum who is best positioned to beat Trump, or if they are willing to roll the dice and court disaster simply to protect the status quo for the political and financial establishment of this country.”

Mr. Weaver also dismissed the notion that his candidate fares better against Mr. Trump only because he hasn’t been under the same white-hot spotlight Mrs. Clinton has faced for years.

“Some people say we do better against Trump because we haven’t faced the Republican attack machine yet. But we’ve been told our goals for the future are utopian, and that our plans would raise taxes on middle class families. We just never thought those attacks would come in a Democratic primary. Yet somehow we keep winning,” he said. “We are the best chance to defeat Trump because people united can never be defeated. That is why we must keep fighting.”

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For her part, Mrs. Clinton still is trying to ignore Mr. Sanders and turn all of her attention to the November battle with Mr. Trump.

But even the Clinton campaign has been forced to concede that it is neck and neck with Mr. Trump in key battleground states, and actually trailing in Ohio, according to recent polling. Clinton campaign manager Robby Mook on Wednesday morning sounded an almost desperate tone in a fundraising email, saying Mrs. Clinton needs more money or she risks falling even further behind against Mr. Trump.

“I can’t say this enough times: These polls don’t predict the future. We can change them by making sure voters know about Hillary’s vision for our country. But we need to get started RIGHT NOW,” he said before asking supporters to donate to the campaign.

The fundraising pitch cited this week’s Quinnipiac survey of battleground states, which shows Mrs. Clinton leading Mr. Trump by just 1 percentage point in Florida and Pennsylvania and down by 4 percentage points in Ohio.

While those poll numbers surely are alarming for Democrats, it’s now all but certain Mrs. Clinton will be her party’s nominee.

She leads Mr. Sanders among pledged delegates 1,716 to 1,433, according to an Associated Press tally. Among superdelegates, she’s ahead 524 to 40.

It takes 2,383 delegates to seal the nomination.