The American left is very good at turning ferociously on politicians who start ill-advised wars. In Lyndon Johnson’s final years in office, anti-war activists picketed the White House holding signs saying, “Lee Harvey Oswald, where are you now?” A poll found that by 2006, 76 percent of Democrats were open to impeaching George. W. Bush Where the American left is less effective is in preventing politicians from starting those wars in the first place.

Take, for example, the legislation introduced last week by fourteen Senate Democrats (and thirteen Republicans) to levy new sanctions on Iran unless its leaders agree to a final nuclear deal that, among other things, abandons all uranium enrichment. To understand how absurd that standard is, it’s worth noting that Mohammed ElBaradei, former head of the International Atomic Energy Agency, and John Kerry have both said Iran has the right to enrich uranium for peaceful purposes. That right is felt so strongly in Iran that even Mir-Hossein Mousavi, the reformist who had the presidency brutally stolen from him in 2009, has declared that enrichment “is our right and we have no right to backpedal” on it. Iran’s current foreign minister has said that if the U.S. enacts new sanctions, even ones that go into effect at a later date, “the entire [interim] deal [signed last month in Geneva] is dead.” A December 10 report by U.S. intelligence concurred, declaring that “new sanctions would undermine the prospects for a successful comprehensive nuclear agreement with Iran.” White House press secretary Jay Carney has said they would “greatly increase the chances” of war.

So why on earth are fourteen Democrats joining Ted Cruz and Marco Rubio in openly defying the president? In a party that is clearly moving left on economics, how can so many prominent Democratic senators support a bill so widely scorned by Democratic foreign policy experts? Because in recent years grassroots Democrats have turned their attention away from Middle East policy and AIPAC has not.

A few years ago, the backlash against such a bill would have been deafening. In 2007, Hillary Clinton voted to designate Iran’s Revolutionary Guard a terrorist group, a vote some feared gave the Bush administration license to attack Iran. Among her opponents for the Democratic nomination, the response was ferocious. “Instead of blocking George Bush’s new march to war,” declared John Edwards, “Senator Clinton and others are enabling him once again.” Candidate Obama mailed a flier to Iowa voters denouncing Clinton for her vote. Candidate Joe Biden called it a “serious, serious mistake.”

But back then, rank and file Democrats were still shaking with rage over Iraq. Over the last few years, the winding down of George W. Bush’s wars plus the financial crisis and rise of the Tea Party has radically reoriented the left’s focus. It’s a sign of the times that the party’s hottest star, Elizabeth Warren, is an anti-Wall Street crusader with no distinct foreign policy views whatsoever.

All of which is terrific for AIPAC, once compared by its former foreign policy director to “a night flower: it thrives in the dark and dies in the sun.” AIPAC will resist any move toward rapprochement with Iran because a menacing Iran is crucial to its efforts to convince American Jews that, without AIPAC, they’d be living in 1938. Only a mobilized liberal activist base can overcome AIPAC’s influence. But despite the best efforts of MoveOn, CREDO, that doesn’t exist right now.

Among the senators who have cosponsored the new sanctions legislation are Charles Schumer and Kirsten Gillibrand from New York, Chris Coons from Delaware, Richard Blumenthal from Connecticut and Robert Menendez and Cory Booker from New Jersey. These are bright blue states where most Democratic voters would be appalled to know that their senators support a bill that both President Obama and U.S. intelligence believe increases the chances of a third Middle Eastern war. Indeed polls show that Democrats overwhelmingly back Obama’s Iran policy. The more these senators are forced to publicly defend their positions on Iran, the more politically costly those positions will become. What we don’t know is whether the left can generate a movement strong enough to force that public debate.

In 2006, Democrats enraged by Joe Lieberman’s support for the Iraq war denied him their party’s renomination for senate. In 2008, Democrats embittered by Hillary Clinton’s support for Iraq helped orchestrate one of the biggest upsets in presidential history. But they were too late; the damage was already done. The American left is very good at punishing politicians for supporting disastrous wars. Its challenge in 2014 is to show that it can stop politicians from promoting those wars in the first place.