“For four years, when the American people saw Joe Biden, they saw someone who was clearly not capable of making major decisions for the nation,” Schmitt said in a statement. “Yet those closest to the president and the mainstream media did everything they could to hide this truth.”

Schmitt said a group of insiders “were the de facto commander in chief” while Biden, 82, was “sidelined.” Cornyn echoed that, saying the country “depends on having a president who has the mental capacity to do the job, and it’s clear that President Biden did not.”

Cornyn called the alleged effort a “conspiracy between the mainstream media, Joe Biden’s family, and his inner circle,” accusing them of having “lied consistently to the American people about his capacity to make decisions, which are solely vested by the Constitution.”

The lawmakers said they are still finalizing a witness list for what would be Congress’s first formal committee inquiry into Biden’s health and mental fitness.

Their inquiry comes as House Republicans ramp up a parallel investigation into the Biden administration’s alleged concealment of the former president’s condition. Rep. Buddy Carter (R-GA) introduced legislation Thursday to create a select committee that would explore “the potential concealment of information from the American public” about Biden’s health.

The Senate hearing also adds to the political fallout from a related House investigation into the use of autopen technology by the Biden administration to sign executive orders and pardons in his final days. That investigation has also prompted broader concerns about who was making key decisions during Biden’s presidency and whether he remained fully engaged.

House Oversight Committee Chairman James Comer (R-KY) is pursuing interviews with several high-ranking Biden staffers over their alleged roles in masking the president’s decline. Those targeted include Neera Tanden, former Domestic Policy Council director; Annie Tomasini, deputy chief of staff; Anthony Bernal, longtime aide to Jill Biden; Ashley Williams, deputy director of Oval Office operations; and Biden’s personal physician, Dr. Kevin O’Connor.

All five people have made initial contact with the committee through their attorneys since receiving letters from Comer on May 22, a source familiar with the panel’s work told the Washington Examiner on Thursday.

“We expect all witnesses to fully comply with the Committee’s investigation. The Committee will issue subpoenas if necessary,” the source said.

Momentum behind the congressional push to uncover more about Biden’s condition has grown in the wake of the new book Original Sin, authored by CNN’s Jake Tapper and Axios’s Alex Thompson. The book alleges that the White House worked aggressively to shape the narrative around Biden’s health and shield the public from the extent of his decline. It also raised new suspicions surrounding Bernal, Jill Biden’s aide, and the extent to which he was influencing the president’s decision after the book made claims that he wielded “enormous” power in the former administration.

Comer teased on Wednesday that there could be additional subpoenas for the former president and his spouse in the separate House investigation. The senators made no mention of Biden’s autopen in their press release on Thursday.

DEADLINE LOOMS FOR EX-BIDEN AIDES TO COMPLY WITH GOP AUTOPEN INVESTIGATION

Cornyn and Schmitt’s announcement came just hours after White House press secretary Karoline Leavitt called for the former first lady to “speak up” about her husband’s health condition ahead of his Oval Office departure.

“I think anybody looking at the videos and photo evidence of [former President] Joe Biden with your own eyes and a little bit of common sense can see this was a clear cover-up,” Leavitt said during a press briefing.