Joe Biden’s campaign is urging its active social-media supporters to get online and defend the Democratic presidential candidate against what it sees as increasingly aggressive attacks from the surging Bernie Sanders camp.

With less than a week before the Iowa caucuses, the Biden campaign expressed concern on a call to its supporters that Sanders people were “getting ugly” and it had to “step up its game” defending the vice president. The message was confirmed by campaign national press secretary TJ Ducklo.

The Sanders campaign has seen a sudden uptick in support in recent weeks, taking leads in polls in Iowa and New Hampshire, which holds its primary on Feb. 12. Sanders is also leading in California, which provides 10% of the delegates needed to secure the presidential nomination.

Biden continues to hold a sizable lead in national polls.

Sanders’ campaign is also rallying supporters to defend him against what it sees as Biden’s attacks. It sent an email to supporters today asking for donations to counter attack ads planned by Democratic Majority for Israel which is placing $700,000 worth of attack ads against Sanders. The PAC is not affiliated with the Biden campaign.

The email signed by Sanders Campaign Manager Faiz Shakir said Sanders has “a small lead in Iowa” but “outside groups are on the attack and hoping to stop us.”

Sanders’ supporters are markedly more aggressive than those of any other campaign, going to Twitter and Facebook to enthusiastically defend the Vermont senator and sometimes attack the supporters of other candidates. Sanders’ most fervent supporters famously refused to support 2016 Democratic nominee Hillary Clinton when she beat Sanders.

Although with many voters in Iowa undecided before the Feb. 3 caucuses, any one of the top tier candidates — Sanders, Biden, Amy Klobuchar, Pete Buttigieg and Elizabeth Warren — could still claim victory.

But Sanders and Biden have been attacking each other relentlessly, with Sanders’ campaign saying some of Biden’s past statements suggest the former vice president is open to cuts in Social Security, though he has not suggested that this election cycle. Biden’s camp asserted that Sanders had used similar language, which Sanders said had been taken out of context. The spat highlights the fight over older voters, who tend to vote in higher numbers than other demographic groups.

Sanders and Biden represent the two wings of the Democratic Party that are competing for the opportunity to take on President Donald Trump in the fall. Sanders, a self-described “democratic socialist,” is running on a progressive platform advocating Medicare for All and the Green New Deal. Biden offers a moderate path, returning the country to what he considers normalcy after the Trump years.