Peter Beinart asks a great question, but fails to mention the possibility that anti-Semitism, and being accused of being an anti-Semite, are the reasons why Democrats are holding back on vocally opposing a war with Iran.
The people opposed to making a deal with Iran have a lot of money they can throw at politicians. They can fund primaries against candidates or throw a lot of money at an opponent. They can buy a lot of ads and use a great deal of lobbying to get what they want. In the next election cycle, look for a lot of this money to be thrown at any Democrat who supports the President on the Iran deal.
And, as always, look for charges of being anti-Semitic to rain from the skies. That's how it works.