“We have a small lead in Iowa heading into Monday’s caucus,” Mr. Shakir wrote. “But outside groups are on the attack and hoping to stop us. Bernie needs us all if we’re going to fight back and win.”
Past attacks have boomeranged to Mr. Sanders’s advantage. When Hillary Clinton ripped into Mr. Sanders in an interview published last week, her comments led to a rallying of support for his candidacy online.
The group made its ad reservations on Monday, booking more than $680,000 to air only in Iowa starting on Wednesday morning and running just through the caucuses, a significant level of saturation over a short span just before the nominating contest. The buy is heavily weighted toward Cedar Rapids and Des Moines markets, and the group is also airing the ad on cable, including on CNN, MSNBC and ESPN.
News that the group had made reservations was swirling around Democratic circles on Tuesday morning, and the Sanders campaign sent a fund-raising text message to supporters warning that “an outside group is starting a $700,000 NEGATIVE AD CAMPAIGN against Bernie.” The text asked for $2.70 to help push back.
The ads follow a weekend in which Mr. Sanders faced an onslaught of attacks from Democratic rivals. Pete Buttigieg sent a fund-raising email warning his supporters that “Bernie Sanders could be the nominee of our party,” followed by another email casting doubt on his ability to defeat Mr. Trump. Michael R. Bloomberg, who is skipping the early states to focus on Super Tuesday and beyond, also took a jab at Mr. Sanders’s political beliefs, saying that Mr. Bloomberg was the only Jewish candidate in the race who “doesn’t want to turn America into a kibbutz.”
One reason for Mr. Sanders’s surge in Iowa is the inability of moderates to coalesce behind one candidate. It is unlikely that the negative attacks on Mr. Sanders will unite moderates at this point. But it could spur them to caucus out of fear that not doing so could lead to a victory for Mr. Sanders, who benefits from a loyal and unwavering bloc of supporters.
Any anti-Sanders effort, however, is also likely to galvanize his supporters, who often rush to his side at any sign of adversity.
Nick Corasaniti contributed reporting from Des Moines.