Charles Krauthammer, a longtime Fox News contributor and Pulitzer Prize winner, died Thursday at the age of 68. The Harvard-trained psychiatrist's death was expected, as he released a heartbreaking letter earlier this month detailing his cancer diagnosis.

Krauthammer said on June 8 that his cancer had returned and was "aggressive and spreading rapidly." His doctors estimated he only had weeks to live.

In recent years, Krauthammer was best known for his nightly appearance as a panelist on Fox News' "Special Report with Bret Baier" and as a commentator on various Fox News shows.

Friends, fans, journalists and politicians took to Twitter Thursday to mourn Krauthammer.

"Charles Krauthammer will long be remembered for his eloquence, his triumph in hardship, and his countless contributions to American political thought," Vice President Mike Pence tweeted. "Karen & I send our condolences and prayers to his family and friends during this difficult time."

"For decades, Charles’ words have strengthened our democracy," former President George W. Bush said in a statement. "His work was far-reaching and influential - and while his voice will be deeply missed, his ideas and values will always be a part of our country. We send our thoughts and prayers to Robby, Daniel, and the entire Krauthammer family."

House speaker Paul Ryan, R-Wis., labeled Krauthammer as "one of the great thinkers of our time."

"A giant in his intellect and his character," Ryan wrote. "A good and gracious man. And a dear friend. This is such a loss. Our prayers are with his family, friends, and colleagues."

Senate Majority Leader Mitch McConnell, R-Ky., said he was "deeply saddened" by the news in a statement.

"Just because this news didn’t come as a surprise does not make it any easier to face," McConnell said. "As a writer, speaker, and commentator, Charles served our society as a public intellectual in the truest sense. His pen functioned like a lighthouse, helping all of us see more clearly and reason more thoroughly through the most important issues that our nation faced."

British journalist Piers Morgan described Krauthammer as a "brilliant political analyst." Morgan said he "loved" Krauthammer's "commentaries, which were always fair, balanced, intelligent & insightful. A great loss in these echo chamber times."

"A huge loss to vigorous and civil debate on public policy," Marty Baron, executive editor of The Washington Post, where Krauthammer's work as a columnist earned him a Pulitzer, tweeted.

Conservative New York Times columnist Ross Douthat said there was "No greater master of the form" than Krauthammer.

Bret Stephens, also of The Times, described the loss of Krauthammer as "irreplacable."

The Daily Beast's Lachlan Markay shared a short anecdote on Twitter about the only time he met Krauthammer in a Fox News greenroom in Washington, D.C.

"I walked in to find him and [Jonah Goldberg] in deep conversation about Italian fascism. As a young conservative, it was basically like getting free tickets to see my favorite band," Markay said.

"God rest the soul of the great Charles Krauthammer the guru of intellect honesty truth logic & commonsense," wrote Ted Nugent.

Megyn Kelly said, "We've lost a national treasure."

The Washington Nationals tweeted that Krauthammer was "One of baseball's greatest fans. ... He was loved and admited by many and will be truly missed here at Nationals Park."

The ballpark held a moment of silence in his honor Thursday night at the Nationals' game against the Baltimore Orioles.

"Many of us in political media stand on the shoulders of giants," pollster Frank Luntz tweeted. "Charles Krauthammer was one of the tallest. R.I.P."