President Donald Trump may have backed away from a critical promise to U.S. Attorney Preet Bharara, Democratic former New York Rep. Elizabeth Holtzman said Monday.

"From my point of view, it appeared that the president had given his word to Preet, saying you're going to stay on. We didn't hear that from the president, we heard it from Preet," Holtzman, also a former district Attorney of Kings County, New York, said on "Squawk Box."

"In politics, your word is critical," she said.

Bharara, U.S. attorney for the Southern District of New York, whom many regard as the Justice Department's most powerful prosecutor, said the Trump administration fired him on Saturday after he refused to step down.

Preet told reporters late last year that Trump had asked him to stay in his post.

@PreetBharara tweet.

Bharara and his office are known for their successful prosecutions of Wall Street figures, such as securing the guilty plea of Bernie Madoff's brother, Peter, for his role in the infamous Ponzi scheme.

Holtzman said it is "hard to think" the Trump administration would fire Preet because he could undermine the president in the future.

"Nobody really knows the answer. Could've been to get rid of Preet? Could be,'" she said.

The White House has not officially commented on Bharara's departure.

— Reuters and CNBC's Christine Wang contributed to this report.