(Richard Shiro/AP)

Donald Trump is not ruling out a run for president as an independent if things go south for the front-runner in the Republican race.

"I'm going to have to see what happens," he told George Stephanopoulos in an interview that aired Sunday on ABC's "This Week." "I will see what happens. I have to be treated fairly."

Trump was responding to a Wall Street Journal report that Republican operatives are considering banding together donors from the other GOP campaigns in a bid to knock Trump off the top spot.

Their efforts are taking on increased urgency as the first-in-the-nation Feb. 1 Iowa caucuses near and Trump leads the crowded GOP field for the fourth consecutive month, according to a new national Washington Post-ABC News poll. Some candidates, including Ohio Gov. John Kasich and former Florida governor Jeb Bush, also are showing more willingness lately to directly attack Trump.

On Sunday, Trump said his measure of whether he would support the eventual GOP nominee is whether he is treated "fairly" in the campaign, a line he often uses when talking about a third-party run.

"When I did this, I said I have to be treated fairly," Trump told Stephanopoulos. "If I’m treated fairly, I’m fine. All I want to do is [have] a level playing field."

Trump has given competing signals throughout the campaign about whether he would run as a third-party candidate. He didn't raise his hand in the Republican Party's first presidential debate, in August, when moderators asked the candidates whether they would pledge to support the eventual GOP nominee. But in September, Trump signed a pledge saying he would support the eventual nominee — instead of opposing him or her in a third-party run.

If Republican operatives succeed in knocking Trump off his perch, their plan could backfire if he then runs as an independent.

A July Washington Post-ABC News poll found that in a hypothetical general election matchup between Bush and Democratic front-runner Hillary Clinton, Clinton led 50 percent to 44 percent. But throw a third-party Trump run into the mix, and the poll found that he would tear support away Bush and give Clinton a 16-point lead.