Manchin rips Cornyn for filibuster flip as Texas GOP runoff looms

"These extreme election-year politics that put party power over everything else are why Americans are sick and tired of the duopoly of the two-party system of Democrats and Republicans," Manchin added.

"For many years, I believed that if the U.S. Senate scrapped the filibuster, Texas and our nation would stand to lose more than we would gain," Cornyn wrote. "But when the reality on the ground changes, leaders must take stock and adapt."

Manchin, a former Democrat-turned-Independent who passed on running for re-election in 2024, alleged that Cornyn personally reached out to him after he defeated Democrats' gambit to nix the 60-vote rule under former President Joe Biden.

In early 2022, Manchin supplied the critical vote alongside former Sen. Kyrsten Sinema, I-Ariz., and Republicans to tank a Democratic-led effort to abolish the filibuster and pass so-called voting rights legislation.

"It's deeply disappointing to see that Senator Cornyn is now willing to scrap the very rule he once praised and personally thanked me for defending," Manchin wrote.

At that time, Cornyn urged Democrats to preserve the filibuster while Republicans were serving in the minority.

"Power is fleeting and at some point the shoe will always be on the other foot," Cornyn said in a floor speech. "Liberal activists may like the idea of nuking the filibuster today, but they'll soon find themselves ruing the day their party broke the Senate."

Cornyn denied Manchin's account Thursday. He has also argued that his reversal on the Senate filibuster was not aimed at winning Trump's endorsement. 

"There's no Joe Manchins left in the Democratic Party and no Kyrsten Sinemas… this is an entirely different circumstance, dealing with Democrats who will not negotiate or consider anything that President Trump or Republicans want," the Texas Republican told reporters. "We can either accept that or we can fight back, and I think we should fight back."

"Senator Cornyn is one of 53 Republican senators, and the opposition to nuking the filibuster runs very, very deep in our conference," Thune told reporters.

Manchin has continued to sing the filibuster's praises in his retirement, arguing that the 60-vote threshold protects the minority party and forces legislation to be bipartisan.

"The filibuster - the soul of the Senate - has preserved the Senate's role for nearly 250 years as the institution that cools passions, protects minority voices, and demands consensus," Manchin said. "America was built on institutions designed to resist political convenience, not surrender to it."

Fox News Digital reached out to Cornyn's office for comment.