'Worst president': Donald Trump reacts to President Joe Biden dropping out of 2024 election

"Biden was not fit to run for President," he wrote.

Former President Donald Trump swiftly reacted to President Joe Biden's withdrawal from the 2024 election, saying his former opponent was "not fit to run."

Trump issued his statement on Truth Social, writing, "Crooked Joe Biden was not fit to run for President, and is certainly not fit to serve - And never was!"

He continued, "All those around him, including his Doctor and the Media, knew that he wasn’t capable of being President, and he wasn’t."

Trump went on to bring up some of his regular campaign talking points, slamming Biden over the border before adding, "We will suffer greatly because of his presidency, but we will remedy the damage he has done very quickly. MAKE AMERICA GREAT AGAIN!"

At the start of his statement, Trump also called Biden "the Worst President, by far, in the History of our Nation."

The former president's campaign immediately began a new fundraising push following Biden's announcement, writing, "But RIGHT NOW, he just quit the race in COMPLETE DISGRACE!"

House Speaker Mike Johnson wrote on X, "At this unprecedented juncture in American history, we must be clear about what just happened. The Democrat Party forced the Democrat nominee off the ballot, just over 100 days before the election."

He also called for Biden to resign.

"If Joe Biden is not fit to run for President, he is not fit to serve as President. He must resign the office immediately. November 5 cannot arrive soon enough," Johnson posted on X.

Sen. Josh Hawley quickly reacted to the Biden news, calling for the president to step down.

"Then RESIGN your office. If you can’t run a mere political campaign, you can’t be President," he wrote in response to Biden's statement.

House Majority Leader Steve Scalise wrote on X, "Democrat party bosses just proved that they have absolutely no respect for their own voters. After lecturing others about democracy, they just forced Joe Biden off the ticket—trashing the primary choice of 14 million of their own voters.

Senate Minority Leader Mitch McConnell said in a statement, reacting to Biden's announcement, "For four years, the American people have faced historic inflation at home, chaos at the border, and weak leadership on the world stage. Our nation is less prosperous and less secure than it was in January, 2021. We cannot afford four more years of failure.

“Unfortunately, the Democratic Party has been busy in recent weeks trying to upend the expressed will of the American people in primary elections across the country. Washington Democrats have not proven themselves any more capable than the President of delivering the secure borders, safe streets, and stable prices that working families deserve. They are selling open borders, higher prices, climate radicalism, and soft-on-crime policies, and the American people are not buying," McConnell's statement concluded.

Trump campaign advisers Chris LaCivita and Susie Wiles, on behalf of the former president's campaign, issued a statement slamming Biden and Vice President Kamala Harris.

"Crooked Joe Biden is a complete fraud and a disgrace to our Country," they said in their statement. …

They continued, "And during this entire term, Kamala Harris – as well as every other Democrat in Washington, sat by and did NOTHING. They are all just as complicit as Biden is in the destruction of our once-great Nation, and they must all be thrown out of office."

They also questioned Biden staying in office following his decision not to run.

"The question then to Kamala Harris is simple: knowing that Joe Biden withdrew from the campaign because of his rapidly deteriorating condition, does Harris believe the people of America are safe and secure with Joe Biden in the White House for six more months?"

In a statement on Sunday, Biden announced he was withdrawing from the 2024 election.

"It has been the greatest honor of my life to serve as your President. And while it has been my intention to seek reelection, I believe it is in the best interest of my party and the country for me to stand down and to focus solely on fulfilling my duties as President for the remainder of my term," he said in his statement.

The president immediately threw his support behind Vice President Kamala Harris for president.

"My very first decision as the party nominee in 2020 was to pick Kamala Harris as my Vice President. And it’s been the best decision I’ve made."

"Today I want to offer my full support and endorsement for Kamala to be the nominee of our party this year," Biden said in a post on X. "Democrats — it’s time to come together and beat Trump. Let’s do this."

Biden said he plans to address the nation later this week with more details on his decision to leave the race.

-ABC News' Soo Rin Kim contributed to this report.