Rep. Claudia Tenney: Biden 'has been isolated from his advisers and asleep at the wheel'
Blames double standard, was facing ethics investigation into affair allegations
Nadia Schadlow is third national security official to leave post since John Bolton became national security adviser
Senior Republican is member of tax-writing Ways and Means Committee
Cohn: 'I have come under enormous pressure both to resign and to remain in my current position'