Progressive historian David Garrow compared former President Barack Obama to a sellout for how he changed as a person and politician in Chicago as he moved to the Senate and the White House.

"As a progressive Democrat, I'm disturbed and perplexed that he has changed so dramatically over the last 15 years," Garrow told Fox News host Tucker Carlson Monday evening.

"We look at his record back in Illinois in the 1990s up through 2002 — you would think we knew who he was, but he has turned out to be a different person then we would have projected."

Garrow's new book "Rising Star" looks at Obama's formative years. At the end, Garrow writes a lengthy epilogue with his conclusion — one that he said he did not expect to reach.

While Obama lived in Illinois, he had been an "outspoken proponent" of single-payer health coverage, a critic of the Patriot Act and the intelligence community and the CIA.

"So the Obama that we have seen as president has been a dramatically different person up through 2002," Garrow said.

"Barack lived a very modest, humble, middle-class life. Now here we are in 2017 and we see him hobnobbing with celebrities and musicians and billionaires, getting $400,000 a speech."

Carlson asked if Obama was a good person. Garrow said Obama used to be, but was overtaken by his "desire to succeed."

"The need to become president changed him — fundamentally changed him. And that to me is a fundamentally sad story," Garrow added.

The author dismissed a bad review from the left-leaning New York Times by saying the paper defended Obama at the expense of progressivism.

"I think in today's culture and today's politics, Tucker, unfortunately, we sometimes see partisanship trumping — a verb I like — trumping professionalism. I'm an academic, a scholar, historian. It's sad the New York Times gives in to partisan fervor, but I am playing a long game. I'm a scholar and this book will be the authoritative account of Barack Obama's prepresidential life, I believe for decades to come," Garrow finished.