A former Green Party candidate for president took aim at both parties Tuesday, as critics denounced Howard Schultz's potential third-party presidential campaign as a spoiler for Democrats.

Former 2012 and 2016 Green Party presidential candidate Jill Stein, in an appearance on Fox News, said both parties are "working for corporate masters and the fossil fuel giants, and the big banks, and Wall Street, and the war machine. That is not my opinion, that Democrats are the party of diversity. Voters deserve more choices."

Schultz, a former Starbucks CEO who is weighing a 2020 presidential run as a "centrist independent," pushed back Tuesday against claims he would be a spoiler for the Democratic candidate, whoever that may be.

"I can't think of anything that is a more quintessential expression of our democracy than providing the American people with a choice that doesn't have to be binary between the Republican and the Democrat," Schultz told NPR. "Why should the American people not have the choice of someone who is saying, 'I'm not embedded with either party'?"

Stein, a physician who said she has no plans to run in 2020, also expressed support for more choices for voters.

"In the last election we saw over 100 million voters, the largest bloc of voters, not voting because they are not being represented by the system. We need noncorporate choices and people-power choices," she said. "And we can have those choices without fear of splitting votes. There is a win-win solution here. It is called ranked-choice voting."

Ranked-choice voting, which Maine used for a federal election for the first time in 2018, allows voters to rank their preferred candidates. If no candidate receives a majority, the candidate with the fewest first choices is eliminated and voters who liked that candidate the best have their ballots instantly counted for their second choice, which ensures a voter can select a third-party candidate without fearing that candidate will take a vote away from for whichever major-party candidate aligns with that voter's views.