This number may be the biggest 'alarm siren' in Hillary Clinton's brutal summer

AP/Jay LaPrete

Former US Secretary of State Hillary Clinton.

Support for former US Secretary of State Hillary Clinton's presidential campaign tumbled in a new Washington Post-ABC News poll published Monday.

And the drop was sharpest in one of Clinton's core constituencies: Democratic women.

Among all Democratic-leaning voters, Clinton's Democratic primary support has dipped from 63% to 42% since July.

But among female, Democratic primary voters, her support plunged even more: from 71% to 42% - or to where it's about even with men.

"Even if still in a strong position, Clinton's trajectory leaves no question that she has trouble," the pollster's write-up of the results said. "Her support in the primary has tanked in particular among women, previously a mainstay of her candidacy, from 71 percent in July to 42 percent now."

The Washington Post's Karen Tumulty described Clinton's drop among women - and Democratic women especially - in even starker terms, including "rapid erosion," "steep decline," an "alarm siren" for her campaign, and "the main force driving the poll's overall numbers" that showed her primary support dropping by one-third since July.

That drop follows a difficult summer for Clinton's campaign, notably the controversy over her email use as secretary of state.

Clinton exclusively used a personal email server for her government business, which she insists was allowed. But the choice has nevertheless raised questions about the security of classified material and whether all government-related messages were turned over to government archivists as required.

"The poll suggests that the historic significance of Clinton's campaign, which holds the prospect of electing the nation's first female president," Tumulty wrote, "is being overtaken by other forces."

REUTERS/Dominick Reuter

US Sen. Bernie Sanders (I-Vermont).

As Clinton's campaign has stumbled, US Sen. Bernie Sanders (I-Vermont) has surged.

Sanders, running as a Democrat, has embraced an outspoken message that appears to be resonating among Democratic voters. He has overtaken Clinton in Iowa and New Hampshire, according to a recent poll, though he still lags in South Carolina and nationally.

Vice President Joe Biden is also looking at potentially jumping into the race, which has only amplified the public airing of some Democrats' discontent with the Democratic front-runner.

For her part, Clinton dismissed the poll results as not especially significant, particularly given the fact that the first votes won't be cast until early next year. Speaking Monday at a press conference in Cedar Falls, Iowa, Clinton said she has "been in and around enough campaigns to know that there's an ebb and flow" in polls, according to The Post.

"Polls go up and down," Clinton said. "I feel very confident about where we are in the campaign and very committed to doing everything I can to make my case as effectively as possible to women and men, and I think that will be successful."

Check out The Washington Post's write-up of the poll >>