WASHINGTON — Donald Trump is leading the Republican presidential race by so much — and at so little cost — that he feels “guilty” about it, he said Sunday.

“Look, I was going to have $35 or $40 million spent by now. I haven’t spent anything. I almost feel guilty . . . I’m leading by, as you all say, a lot,” he boasted on CBS’s “Face the Nation.”

The billionaire New York developer, whose campaign is self-funded, has relied on TV appearances and coverage of his tweets, rallies and controversial statements — a strategy that has allowed him to dominate the media landscape on the cheap.

“You can take any poll and I’m winning by a lot. I don’t think I need the ads. But I’m doing them . . . I was going to have at least $35 million spent as of Jan. 1 — I spent almost nothing,” he said.

Trump — who promised to be a better “cheerleader” for the country as president than President Obama — talked up his chances at the caucuses in Iowa, where he is in a tight race against Sen. Ted Cruz of Texas.

“I want to win Iowa. I do great with evangelicals, and I do great with the Tea Party, and I’m doing great. I have a real good feeling with Iowa,” he said.

Meanwhile on Sunday, Trump ratcheted up his attacks on Democratic front-runner Hillary Clinton, saying her political judgment has had grim consequences.

“She has caused death. She has caused tremendous death with incompetent decisions. I was against the war in Iraq. I wasn’t a politician, but I was against the war in Iraq. She voted for the war in Iraq.”

Also, he insisted, there’s nothing he can do about recently being featured in a recruiting video by an al Qaeda offshoot in Africa, while still insisting Clinton is a liar for claiming his comments about Muslims are being used by ISIS to enlist supporters.

“It wasn’t ISIS and it wasn’t made at the time, and she lied,” Trump told “Fox & Friends” on Sunday.

On Twitter, he reiterated his point — and also took a swipe at Bill Clinton.

“Hillary Clinton lied last week when she said ISIS made a D.T. video. The video that ISIS made was about her husband being a degenerate,” Trump wrote on Twitter.

Trump, who has proposed a ban on Muslims coming into the United States, said he has no plans to tone down his rhetoric.