Presidential incapacitation, either anticipated and short term or unexpected and longer term, can be handled by a transfer of power to the Vice President under the procedures of the 25th amendment.
But if a member of Congress becomes incapacitated, there is no legal avenue to replace that person.
Recently, many in Congress have been trying to deal with the issue of members unable to come to Congress, either due to sickness or quarantine, or because hygiene recommendations or logistical issues makes showing up at Congress difficult or impossible. A separate issue, however, is what to do if a member is so sick or severely incapacitated that they could not perform the duties of the job (either remotely or in person).
Congressional history is filled with examples of members who have had severe, debilitating illnesses. Senator Karl Mundt of North Dakota suffered a stroke and was absent from the Senate for the final three years of his term. Illness forced Senator Carter Glass of Virginia to be absent from the Senate for nearly two years. In the case of Glass, citizens of Virginia petitioned courts to remove Glass from his seat, as it was effectively vacant. The court refused to take action.
Never has a sitting member of Congress been removed from office due to incapacitation.
The closest instances were two cases where members elected to office while incapacitated or missing and presumed dead were voted in on the November ballot, but then were ultimately not seated in the next Congress and their seats were then declared vacant.
Representative Gladys Noon Spellman of Maryland suffered a severe heart attack and went into a coma days before the 1980 November election. Spellman won the election, but was unable to show up to be sworn in to the new Congress in January. Eventually, her seat was declared vacant, a special election was held, and current House Majority Leader Steny Hoyer was elected to fill the remainder of her term.
In October of 1972, House Majority Leader Hale Boggs was campaigning in Alaska with Rep. Nick Begich. Their plane crashed in a still unknown location, and they were both presumed dead. Both men were re-elected on the November ballot, but in January, when neither were able to be sworn in to take their seats in the new Congress, their seats were declared vacant and special elections were held to replace them.
Again, aside from these instances of members unable to show up to a new Congress to be sworn in, no sitting member of Congress has ever been replaced for incapacitation.
Congress does have the power to expel its members, but has never done so for medical reasons, and it would seem unlikely that they would do so for a member who had done no wrong and who could not automatically return when recovered.
The issue of incapacitation of members of Congress came up in the aftermath of 9/11. The Continuity of Government Commission considered scenarios where many members of Congress were dead or incapacitated, and the inability to replace these members might prevent Congress from functioning. The problem of mass deaths affected the House, as it has no way to quickly replace deceased members. The problem of mass incapacitations affected both the House and Senate, as those bodies could not function if large numbers of their members were so incapacitated they could not perform their jobs.
In today’s COVID-19 crisis, we hope that we do not face such a dire situation. But what if ten or 20 or 50 members of Congress are so sick that their seats are effectively vacant for significant periods of time? This could result in the effective change of majority. And even remote voting procedures would not alleviate this problem.
The office of the presidency has protection against a president taken down by illness. The Congress does not.