Sen. Thom Tillis (R-NC) wrote a letter Friday to his Senate Republican colleagues about the implications of President Joe Biden’s performance at Thursday’s debate.

In the letter, obtained by the Hill, Tillis suggested that Biden, 81, should step aside “for the sake of the country” and that if he is “unaware of his own unfitness to continue serving,” the Cabinet should consider invoking the 25th Amendment, by which members can vote to transfer authority to the vice president.