Republican presidential front-runner Donald TrumpDonald John TrumpDems flip Wisconsin state Senate seat Sessions: 'We should be like Canada' in how we take in immigrants GOP rep: 'Sheet metal and garbage' everywhere in Haiti MORE said on Sunday that it's "highly highly unlikely" that he will launch a third-party campaign.

Trump added, however, that the Republican National Committee (RNC) is not honoring the pledge he signed last year.

"The pledge is the pledge," Trump said on "Fox News Sunday."

"I signed a pledge. I'm a Republican. I'm the leading Republican by a lot and that's where I want to be. I don't want to run as an independent. I'm not going to be doing that. I think it's highly highly unlikely."

But Trump said he's not being treated fairly by the RNC. For example, he said that during the debates, the room is "stacked" with donors and special interests.

Trump stressed that he is self-funding his campaign, saying he's not controlled by special interests, donors or lobbyists like many of his competitors.

"I'm putting up millions and millions and millions of dollars, like throwing it out the window. So far it's worked. So far I'm also spending a lot less money than all these other people...and I'm leading by a lot and they're losing by a lot," he said.

"It's very unfair what the RNC is doing," he said. "They're stacking the room and that's unfair." 

Trump first threatened a third-party run last summer in an exclusive interview with The Hill.