LAS VEGAS—Sen. Elizabeth Warren said a new super PAC supporting her presidential candidacy is a political necessity, marking an about-face for a candidate who has long sought to distance herself from outside money in politics.

Asked Thursday if she wanted the super PAC backing her to stand down, Ms. Warren told reporters: “So, here’s where I stand: If all the candidates want to get rid of super PACs, count me in. I’ll lead the charge. But that’s how it has to be. It can’t be the case that a bunch of people keep them and only one or two don’t.”

The Massachusetts Democrat’s comments come after a new unlimited-money group, Persist PAC, began airing about $1 million in Nevada ads leading up to the state’s caucuses Saturday. The super PAC was created Tuesday, according to paperwork filed with the Federal Election Commission. The ad echoes Ms. Warren’s campaign message that she fights for working people.

The new group has the potential to undercut one of the pillars of Ms. Warren’s campaign—that she is different from her top rivals because she doesn’t have outside help. Now, Persist PAC is spending more on Nevada ads than any other outside group involved in the race, data from ad tracker CMAG/Kantar shows.

Ms. Warren’s remarks Thursday about the PAC came after Wednesday night’s presidential debate in Las Vegas—with her performance widely praised—and her shift in position on PACs follows a disappointing finish in New Hampshire’s primary earlier this month.