The problem is, Nixon resigned after Barry Goldwater (and a couple of others whose names I can't recall at the moment - another Senator, I believe, and a Representative) told him that (i) he had lost virtually all of his support in Congress, (ii) there were more than enough votes in the House to impeach, and (iii) there were more than enough votes in the Senate to convict.
Without enough votes in the Senate to convict, Obama will never resign.
Obama’s hubris will not allow him to resign, I know that. Raising the question of whether or not Obama should resign would raise the question of the constitutionality of his offenses and force some of the Democrat Senators to answer questions regarding the constitutionality of he government over reach.