Republican frontrunner Donald Trump's most recent attack on Democratic frontrunner Hillary Clinton is ironically reaping big rewards for Clinton's campaign.

Trump said last Tuesday that the only thing keeping Clinton afloat in the race was a so-called "woman's card," a remark that quickly spawned the #WomanCard hashtag on Twitter. He also argued that Clinton's lead on the Democratic side of the presidential race would be greatly diminished if she were a man. This so-called "woman's card," however, is actually having the opposite effect.

Clinton's campaign has raked in a whopping $2.4 million during the three days after Trump's remarks, the New York Times reported, marking her campaign's "best fundraising haul yet."

For her part, Clinton used Trump's own remarks against him, including a tweeted campaign ad that turned "woman card" into a message of empowerment:

His remarks also inspired a wave of fresh Clinton campaign merchandise, including the Official Hillary for America "Woman Card":

HillaryClinton.com

Trump resisted the attack during an interview with Fox News' Sean Hannity on Sunday. Instead of backtracking, Trump doubled down.

"She's playing the women's card like I've never seen anybody play it," he insisted. "There's no question about it. If she were a man, I don't think she'd get 5 percent of the vote."

This isn't Trump's first attempt at undermining Clinton's campaign.

Even Clinton's Democratic rival, Vermont Sen. Bernie Sanders, came to her defense following Trump's criticisms of Clinton for using the restroom during a televised debate in December.

"I don't know how to break this to you," Sanders told a crowd of supporters in Iowa at the time. "Donald Trump is very upset. I don't know what his relationship with women has been like, but he has just discovered that women go to the bathroom and it's very upsetting for him!" See Sanders' comments in the video below.