President Joe Biden announced Sunday he would not seek reelection this year and instead endorsed Vice President Kamala Harris to take on GOP presidential nominee Donald Trump , likely shutting down any talk of an open convention at the Democratic nominating convention next month.

My fellow Democrats , I have decided not to accept the nomination and to focus all my energies on my duties as President for the remainder of my term, Biden wrote in a post on X, formerly called Twitter.

My very first decision as the party nominee in 2020 was to pick Kamala Harris as my Vice President. And its been the best decision Ive made, he continued. Today I want to offer my full support and endorsement for Kamala to be the nominee of our party this year. Democrats its time to come together and beat Trump. Lets do this.

Bidens surprise announcement Sunday upended what was already a turbulent presidential race following Trumps recent assassination attempt and Bidens poor performance against Trump in a debate last month, opening a wave of Democratic calls for him to step aside for a younger nominee.

Some Democratic lawmakers who expressed concerns with Bidens physical fitness and his ability to beat Trump in November had called on the party to hold an open convention, giving Harris and other potential presidential contenders an equal opportunity to lead the party in the future. Others, however, argued that Harris had earned the job and that denying the first Black vice president the nomination would send the wrong message to Black voters.

In a statement on Sunday, Harris praised Bidens love of our country, and vowed to win the Democratic nomination and unite her party going forward.

I am honored to have the Presidents endorsement and my intention is to earn and win this nomination, Harris said. Over the past year, I have traveled across the country, talking with Americans about the clear choice in this momentous election. And that is what I will continue to do in the days and weeks ahead. I will do everything in my power to unite the Democratic Party and unite our nationto defeat Donald Trump and his extreme Project 2025 agenda.

Bidens support for Harris, 59, will make challenging her harder for other Democrats potentially interested in doing so. The Democratic Party bench includes several prominent candidates, including Michigan Gov. Gretchen Whitmer, California Gov. Gavin Newsom, Pennsylvania Gov. Josh Shapiro, and Transportation Secretary Pete Buttigieg, among others.

Republicans, meanwhile, previewed their lines of attack against Harris if she becomes the Democratic presidential nominee in their statements following Bidens announcement on Sunday. They focused on highlighting Harris previous statements defending Biden from questions surrounding his age, as well as her being appointed by Biden to work with Central America to deter migration at the southern border.

For four years, those closest to Joe Biden, including Kamala Harris, knew he was not fit to be president and lied to the American people in an effort to hold onto their power, Sen. Rick Scott (R-Fla.) said in a statement. It was painfully clear that Biden wasnt competent for office going way back to 2021.

It is not just Biden-Harris who have been polling so poorly, it is their entire administrations failed, extreme agenda that is crushing hard-working families, causing record inflation, the worst border crisis in our lifetime, and chaos around the world, House Majority Leader Steve Scalise (R-La.) said in a separate statement.