JERUSALEM (Reuters) - When Prime Minister Benjamin Netanyahu accepted an invitation to address the U.S. Congress two weeks before an Israeli election, there appeared to be little downside. As the appointment draws near, the potential pitfalls are growing.

The primary objective was to warn U.S. lawmakers about the risks of a deal on Iran's nuclear programme and keep alive the possibility of further sanctions on Tehran, frustrating President Barack Obama's long-running policy of negotiation with the Islamic Republic.