Bernie Sanders is cutting into Hillary Clinton’s once-commanding lead among female voters, threatening a key element of her campaign firewall, according to a poll released Wednesday.

The former secretary of state holds a slim edge nationally, 44-42 percent, over Sanders in the Democratic presidential race, according to a Quinnipiac University poll.

Clinton led among female voters, 47-41 percent, according to the poll, taken between Feb. 10 and Monday.

In the previous Quinnipiac poll, taken between Feb. 2 and 4, Clinton’s lead among Democratic women was 48-38 percent.

Clinton is counting on female and African-American voters to fend off Sanders’ challenge, which is fueled by younger and more enthusiastic supporters.

Democrats caucus Saturday in Nevada, where Clinton also holds a razor-thin lead, 48-47 percent, according a CNN poll also released on Wednesday.

On the Republican side, Donald Trump remains the runaway national leader, according to Quinnipiac.

He had 39 percent support, far ahead of Marco Rubio at 19 percent, Ted Cruz at 18 percent and both John Kasich and Jeb Bush, who were in single digits.

Trump also held a commanding lead in South Carolina, where GOP voters go to the polls Saturday.

He had 33 percent to Rubio’s 16 percent, while Cruz and Kasich were both at 14 percent and Bush had 9 percent, according a poll by the American Research Group.