US politicians traditionally hail Washington's close partnership with Israel, and assuring those ties will continue has become a campaign trail standard.
But O'Rourke said that while the relationship is crucial, "it must be able to transcend a prime minister who is racist as he warns about Arabs coming to the polls (and) who wants to defy any prospects for peace as he threatens to annex the West Bank."
Speaking on Sunday at an event in Iowa City, the Texas Democrat accused Netanyahu of siding with a "far-right racist" party in a bid to gain a fifth term in power.
"I don't think that Benjamin Netanyahu represents the true will of the Israeli people or the best interests of the US-Israel relationship, or any path to peace" for Israelis and Palestinians, added O'Rourke.
Sanders, a liberal US senator who is Jewish, told a weekend town hall in Iowa that he believes Netanyahu is "an extreme right-wing leader in Israel."
"I do not support his policies, and I think that to speak out against Netanyahu is not to be anti-Israel," he said, adding that President Donald Trump's administration ought to pressure Israeli and Palestinian leaders to "negotiate peace agreements in good faith."
US Republicans largely support Trump's embrace of Netanyahu, and Republican Senator John Cornyn attacked the Democratic criticism as "crazy talk."
The Democratic gripes follow Netanyahu's deeply controversial pledge to annex West Bank settlements if re-elected, widely seen as an appeal to right-wing voters as he squares off against centrist former military chief Benny Gantz.
"This provocation is harmful to Israeli, Palestinian, and American interests," warned Pete Buttigieg, the mayor of South Bend, Indiana and a presidential hopeful.
"Supporting Israel does not have to mean agreeing with Netanyahu's politics. I don't."
Since his inauguration two years ago, Trump has moved the Israeli embassy to Jerusalem and announced Washington will recognise Israeli sovereignty over the disputed Golan Heights.
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