The Republican chair of the House Oversight Committee is demanding that former President Joe Biden's White House physician testify before the panel — ramping up a congressional investigation into Biden's age and health.

Rep. James Comer, a Kentucky Republican, sent a letter to Dr. Kevin O'Connor on Thursday subpoenaing him to testify at a June 27 deposition. He said the committee wants details about "your assessment of and relationship with former President Biden."

House Oversight had last month that O'Conner appear for a transcribed interview, Comer wrote, but the doctor's attorneys declined, citing local laws and medical ethics rules that generally discourage physicians from disclosing confidential information about their patients. Comer wrote that those "arguments lack merit." 

The letter to O'Conner said the committee "sought to understand if you contributed to an effort to hide former President Biden's fitness to serve from the American people." 

It cited a memo penned by O'Conner after Biden's physical, describing the then-president as a "healthy, active, robust 81-year-old male, who remains fit to successfully execute the duties of the Presidency."

O'Connor's lawyers, David Schertler and Mark MacDougall, said in a statement to CBS News: "We just received the correspondence and subpoena from the House Oversight Committee this afternoon.  We will review it carefully and respond to Chairman Comer after we have had a chance to do so."

Comer's inquiry into Biden's health has expanded in recent days, as new reporting emerges on Biden's mental acuity while in office and the circumstances of Biden's withdrawal from the 2024 presidential election. Comer sent letters earlier this week asking five members of Biden's inner circle — Ron Klain, Michael Donilon, Bruce Reed, Steve Richetti and Anita Dunn — to testify about the former president's "mental and physical faculties."

Separately, President Trump ordered an investigation Wednesday evening into whether Biden's aides conspired to "abuse the power of Presidential signatures through the use of an autopen to conceal Biden's cognitive decline." Mr. Trump has suggested some presidential actions — including pardons — could be invalidated if they were signed with an autopen machine, an idea some legal experts have doubted.

Biden vehemently denied Mr. Trump's claims in a statement: "Let me be clear: I made the decisions during my presidency. I made the decisions about the pardons, executive orders, legislation, and proclamations. Any suggestion that I didn't is ridiculous and false."