When President Joe Biden announced Sunday he wouldn't seek reelection, he told Americans he believes it would be best for him to "stand down and to focus solely on fulfilling my duties as president for the remainder of my term."

The bombshell news immediately shifted focus to Vice President Kamala Harris, to whom Biden passed the torch by officially endorsing her to be the Democratic nominee, though she has work to do before the party's convention next month.

While he'll no longer be running, and will be a classic "lame duck," he still must deal with pressing issues and his every move likely will continue to be scrutinized and criticized by Republicans.

Prominent GOP leaders who spent weeks calling on Biden to exit the 2024 race are now demanding that he should resign from office, claiming if he's not fit to be a candidate, he's not able to continue as president.

Former President Donald Trump, who last week accepted the Republican Party's nomination, said Biden is "certainly not fit to serve" as he responded to the political earthquake that Biden would no longer be his election rival.

"If he’s not suited to run for office, I’m not sure he’s suited to serve the country, to continue in that capacity," echoed House Speaker Mike Johnson, the most powerful Republican in Congress, in remarks to ABC News on Monday.

Johnson also suggested Harris and the Cabinet explore invoking the 25th Amendment, which controls when a president is deemed unable to discharge the duties of his office.

Some Republicans have also accused Harris of being involved in a Democratic "cover-up" about what they claim is Biden's failing mental fitness.

The pro-Trump super PAC MAGA Inc. already released an ad saying: "Kamala was in on it. She covered up Joe's obvious mental decline. Kamala knew Joe couldn't do the job, so she did it. Look what she got done: a border invasion, runaway inflation, the American Dream dead."

The White House has pushed back that Biden "looks forward to finishing his term and delivering more historic results for the American people."

Biden, who is currently in Delaware recuperating from COVID-19, will face major tests this week as he's promised to make a speech to the nation on his decision to bow out of the race and as he hosts Israeli Prime Minister Benjamin Netanyahu in Washington.

While no date or time has been given on his address on the 2024 race, Biden's remarks will be a significant milestone in his career that spanned from being one of the youngest senators sworn into office to becoming vice president in 2008 and eventually president in 2020 after three previously unsuccessful attempts at the Democratic nomination.

"I think they should expect to hear from Joe Biden what he has carried in his heart for decades, which is a deep belief in the American people. Joe Biden is an optimist, he knows how hard Americans work, he knows what we are capable of," Delaware Sen. Chris Coons, a key Biden ally and co-chair of the Biden-Harris campaign, said on CNN on Sunday.

Coons said the calls Johnson and others for Biden to resign were "ridiculous" and that Biden stepping down from the office now would be a "great disservice" to the nation.

"I think President Biden has shown in recent weeks his ability to handle and manage complex international matters, to continue his lifelong record of service and his legacy of leadership in foreign policy and to make hard calls and tough choices that help make us stronger and safer here at home," Coons said.

"I expect President Biden will finish out his term this year and there are things that remain to be done," Coons said, citing annual spending bills that must pass Congress and Biden's commitment to securing a peace plan for Gaza.

In addition to meeting with Biden, Netanyahu will address Congress on Wednesday and is also expected to meet with Harris.

"This coming week, I know he will be putting his heart in it once again," Coons said of Biden's cease-fire proposal to end the Israel-Hamas war.