Sam Donaldson, ABC's pugnacious White House correspondent since the beginning of the Carter administration, has signed a book contract for about $250,000 with Random House, sources said. The book is due out in the fall of 1986.

Donaldson said a researcher will help him but he will write the book himself.

"I needed a new challenge," Donaldson said. "It's no secret that I've been wanting to leave the White House but that's not in the cards at the moment, and I'm not grousing about that. But, frankly, I needed something new."

Donaldson is best known for his aggressive style, his way of asking questions in a dogged manner reminiscent of both Howard Cosell and a backbencher in the British Parliament. As Ted Koppel once said, "If they hadn't invented television, Sam would go door to door."

Simon & Schuster and Bantam were also involved in the bidding until Random House won the auction late yesterday afternoon.

"Dan Rather's book, 'The Camera Never Blinks,' sold around 100,000 in hardcover and Mike Wallace's book, 'Close Encounters,' sold around 80 or 85 thousand," said Donaldson's attorney, Robert Barnett. "We think we can be even more successful than that."

Random House senior editor Peter Osnos will edit the book. He said, "Sam is obviously a vibrant, lively, well-placed visible person and has a special perspective on the White House."

Donaldson, who also appears regularly on "This Week with David Brinkley" and as the anchor of "World News Tonight" on Sundays, said he doesn't want to write a "political science analysis of the White House's relationship and the press. The book is mostly about me, what I've seen, what I've done, how I got to where I am.

"This reputation of mine, I'd hope that people will leave the book thinking, 'Well, he really is an aggressive guy but nothing in the extreme.' You know, I frankly don't want the words 'Boy Did He Yell Loud' written on my tombstone."