The crowd cheered.
"And let's see what she does," Trump continued. "I have a feeling she will say no, but we'll hold that for the debates. Do me a favour. Keep it within this room?"
After that Montana debate, Warren brushed off Trump's taunts as she had multiple times before, whenever he deployed his derisive "Pocahontas" nickname for her.
Today, Warren called Trump's months-long bluff by releasing a DNA test that suggested she did have a distant Native American ancestor.
Warren also indicated she hadn't forgotten about Trump's promise in July.
"Remember saying on 7/5 that you'd give US$1M to a charity of my choice if my DNA showed Native American ancestry?" she tweeted.
"I remember - and here's the verdict. Please send the [cheque] to the National Indigenous Women's Resource Centre."
The charity she chose is a nonprofit that seeks to protect Native American women from violence.
"Send them your $1M [cheque], @realDonaldTrump," Warren added.
Warren said she took the test because she had "nothing to hide" - then dared Trump to release his tax returns.
Standing on a soggy White House lawn, Trump denied he had ever made the promise.
"Who cares?" he told reporters, when asked if he had heard about Warren releasing the results of her DNA test.
Another reporter brought up his promise of a US$1 million charity donation.
"I didn't say that," Trump said. "Nah, you'd better read it again."
Soon after, the Hill posted a fact-checked headline: "Trump denies offering $1 million for Warren DNA test, even though he did."
Earlier, Kellyanne Conway, counsellor to the president, dismissed Warren's DNA test as "junk science," an early indication that Trump is not likely to follow through on the donation promise he now denies having made.
"I haven't looked at the test. I know that everybody likes to pick their junk science or sound science depending on the conclusion, it seems some days," Conway told reporters. "But I haven't looked at the DNA test and it really doesn't interest me... "
Trump has had a long history of making bold pledges to donate large sums of money to charity without actually delivering on those promises, as the Washington Post's David Fahrenthold uncovered in a series of articles that won the 2017 Pulitzer Prize for national reporting.
Fahrenthold said he was first intrigued by the question after Trump claimed during his presidential campaign he had raised US$6 million for veterans, including US$1 million of his own money.
Over the course of his reporting, Fahrenthold recounted Trump often did not follow through on his promised donations until pressured by the media, as evidenced by exchange in May 2016:
"On the phone, I asked Trump: Would you really have given this money away if I hadn't been asking about it?
" 'You know, you're a nasty guy,' he said. 'You're really a nasty guy'.
"A few days later, Trump held a news conference in Trump Tower, where he answered my other question. Where was the remainder of the money Trump had raised from other donors, four months earlier? Turns out, it had been sitting in the Trump Foundation, unspent. In this news conference, Trump announced that he had given the last of it away - and he lashed out at the media for asking him to account for the money.
" 'Instead of being like, "Thank you very much, Mr Trump," or "Trump did a good job," everyone said : "Who got it? Who got it? Who got it?" And you make me look very bad,' Trump said. 'I have never received such bad publicity for doing such a good job.' "
Immediately after denying he had promised to donate to a charity of Warren's choice, Trump seemed to welcome her as an opponent in 2020.
"I hope she's running for president because I think she'd be very easy," Trump told reporters.
"I hope that she is running. I do not think she'd be difficult at all, she'd destroy our country. She'd make our country into Venezuela. With that being said I don't want to say bad things about her because I hope she'd be one of the people that would get through the process."
