Republicans are planning to make President Biden’s age a central focus of their messaging in the lead-up to the 2024 elections, including in paid advertising, as Mr. Biden appears to be losing support from key Democratic members of the House and Senate after his disastrous debate performance on Thursday. Republicans in Congress may also aid in that effort by pressing aggressively for more answers about the president’s mental fitness. 

A source at the National Republican Congressional Committee, which is charged with electing a GOP House majority, tells the Sun that Democrats will have to answer for their past support for Mr. Biden on the campaign trail. Press accounts in recent days have described leading Democrats as being privately alarmed by the president’s mental decline, even as they publicly describe him as intellectually fit as ever. Congressman Lloyd Doggett on Tuesday became the first House Democrat to call on Mr. Biden to step aside, while two other, Congressman Jared Golden and Congresswoman Marie Perez, say they think Mr. Biden will lose to President Trump in the fall. 

“The rest of the world has seen what Republicans have been talking about for the past couple of years with regards to his mental capacity. We’ve been pushing to try to get Democrats to answer: is this something they saw themselves?” the source says. 

“This is definitely something that is front-of-mind for all Americans,” the source continues. “We’ve seen it come out in all the public polling, that this is something that is concerning to them.”

It isn’t just Mr. Biden’s age that will be front-of-mind for voters, but also the implications of the president’s potential withdrawal from the race, the source says. On Wednesday, the NRCC released an ad highlighting the possibility of a Harris administration due to Mr. Biden’s apparent frailty. 

“Kamala is Biden’s enabler in chief,” the ad says, including a video of the vice president saying how “proud” she is to serve Mr. Biden. 

“We have a fighter, a leader with skill, vision, determination, and compassion,” Ms. Harris says in the ad, which then cuts to videos of Mr. Biden tripping up stairs, falling off of his bike, turning to shake hands with no one present, and seemingly falling asleep at the recent D-Day anniversary. Music swells to an ominous crescendo as video is played of Democrats praising Ms. Harris, ending with Congressman Jim Clyburn describing her as the “successor” to Mr. Biden. 

“Is this who we want to be president?” the ad asks as it plays some of the more famously odd sayings Ms. Harris has used in the past few years. 

The source at the NRCC tells the Sun that every Democrat, including Ms. Harris, has to answer for why they did not call out Mr. Biden’s obvious physical and mental limitations before they manifested themselves to tens of millions of Americans at the debate.

“No Democrat can leave this episode unscathed. They have all been complicit in hiding this from the American people, and Kamala Harris is at the top of that list. She’s his vice president, she’s been in meetings with him,” the source said. 

“Whether it’s her or Joe Biden on the ballot in November, at the end of the day, they’re hiding from the American people,” the source continued, 

On the elected official side, Republicans in Congress may be able to help in the effort to keep Mr. Biden’s age and physical limitations in the headlines when lawmakers return to Washington. Congressman Chip Roy, a conservative Texas Republican, has already introduced a resolution demanding Mr. Biden be removed from office, at least temporarily. 

Mr. Roy’s resolution, invoking the 25th Amendment, calls on Ms. Harris “to convene and mobilize the principal officers of the Cabinet to declare the President of the United States is unable to successfully discharge the duties and powers of his office.” Under Section Four of the amendment, Ms. Harris has the ability to take over as acting president if a majority of the cabinet agrees to sign a letter with her. 

Ms. Harris has said she will do no such thing. Mr. Roy’s resolution, even if it passes the House, will have no effect in law. 

The president in recent days has refused to take questions from the White House press corps or even speak off-script at a recent Virginia fundraiser. After the debate, Mr. Biden participated in one campaign event in North Carolina before heading to Camp David with his family for the weekend until returning to the White House on Monday night. 

The president delivered short remarks to the press to denounce the Supreme Court’s immunity decision, but took no questions. On Tuesday, at a fundraiser in Virginia, Mr. Biden blamed a heavy international travel schedule for his poor performance despite the fact that he had been at Camp David for a week leading up to the debate, taking regular naps, according to the New York Times. White House staff claimed that the poor performance was caused, in part, by a cold, though the administration confirmed he has taken no medication to deal with it and Mr. Biden himself has made no mention of such an ailment.