Like all forms of collective security, multilateral sanctions require a unanimity rarely achieved in international politics.
Refusing to lift sanctions and adopting tougher rhetoric toward Iran would not be partisan issues. Plenty of Democrats think that those actions are both good politics and good policy.
While Israelis do not care too much about Europeans moral judgments, the E.U. is an important market for them, and European sanctions of any kind would be harmful to Israel.
The United States already has in place comprehensive trade sanctions against Sudan, imposed because of the regime's support for terrorism. While we maintain diplomatic relations, we do not staff our embassy there.
Opponents of U.S. sanctions have made 'unilateral sanctions' their special target. They argue that sanctions observed by many nations would be much more effective. True enough. Far better for trade with an outlaw regime to be restricted by many nations than by just one.