Now the Massachusetts Democrat wants the president to pay up.
On Monday, Warren launched her most aggressive campaign to date rebutting claims the president has made about her heritage. That campaign included the public release of a report by a Stanford genetics professor who analyzed a sample of her DNA and found "strong evidence" that it included Native American ancestry between six and 10 generations ago.
The president does not appear to be in a hurry to make the donation. Asked Monday morning on the South Lawn of the White House about the results of the DNA test, Trump told reporters: "Who cares?"
Trump told reporters later Monday afternoon that his bet only applied if Warren won the Democratic nomination.
"I'll only do it if I can test her personally," the president added. "That will not be something I will enjoy."
Trump discussed the million-dollar wager at a July rally in Montana, while stumping in the state against vulnerable Senate incumbent Jon Tester. Giving himself some wiggle room, he said that if he faced Warren in the 2020 election, he would publicly demand she take a DNA test while on television at a presidential debate.
"We will very gently take that kit, and we will slowly toss it, hoping it doesn't hit her and injure her arm, even though it only weighs probably 2 ounces," Trump said at the rally. "And we will say, 'I will give you a million dollars to your favorite charity, paid for by Trump, if you take the test and it shows you're an Indian.'"
"And let's see what she does, right? I have a feeling she will say 'no,' but we'll hold that for the debates!"
Warren, who has said that she would "take a hard look" at a 2020 run after the November midterms are over, apparently wanted to get the testing out of the way ahead of time.
The Massachusetts Democrat is building out a national political operation and has increasingly gone on the offensive against Trump and Republicans up and down the ballot in recent months.
According to a CNN poll released over the weekend, Warren is favored by 8 percent of voters in the crowded pool of potential 2020 Democratic candidates, trailing former Vice President Joe Biden, and her fellow Sens. Bernie Sanders, I-Vt., and Kamala Harris, D-Calif.
In a post on Twitter, Warren asked Trump to donate the money to the National Indigenous Women's Resource Center, a nonprofit that works to prevent violence against Native women and their children.
"Send them your $1M check, @realDonaldTrump," Warren tweeted.
The organization did not respond to a request for comment.
It is likely that the results of the test will not quell Trump's criticism of the senator, who the president derisively refers to as "Pocahontas."
Warren has said that her great-great grandmother is at least part Native American, which is consistent with the test results' findings. But the test results also leave open the possibility that Warren's Native American heritage goes back 10 generations, which would make her only 1/1,024 Native American.
An earlier effort to tamp down on the president's claim that Warren has benefited professionally from her Native American heritage has not seemed to yield much success.
In September, the Boston Globe published an article, citing more than 100 interviews, that found that ethnicity was "not a factor" in Warren's professional rise. But that has not stopped the president from claiming otherwise.
As recently as Tuesday, at a rally in Iowa, the president said that Warren has "gotten so many advantages" thanks to her claims of Native American heritage. Two days later, on Fox News, the president continued his attack.
"She can't prove anything," Trump said. "She said her mother told her she had high cheekbones therefore she's got Indian blood. You know what? I have more Indian blood in me than she does, I have none unfortunately. I have none."
But Warren appears to be doubling down on the strategy of forcefully rejecting the president's claims. The release of her DNA test came alongside the roll out of a "fact squad" campaign, including its own website dedicated to combating misinformation about her family history.
Correction: The test results leave open the possibility that Warren's Native American heritage goes back 10 generations, which would make her 1/1,024 Native American. An earlier version misstated the figure.