With a tighter-than-expected race for Iowa looming, both Hillary Clinton and Bernie Sanders are trying to woo Democrats by saying they’d be the strongest candidate to go up against Republicans in the fall.

“From the bottom of my heart, above and beyond ideas, if you want somebody who is gonna beat Donald Trump, who is gonna beat the other Republicans, I think Bernie Sanders is that candidate,” Sanders said in Fort Dodge, Iowa.

Republicans seem to be more concerned about facing off against his Democratic opponent.

GOP super PACs and outside groups have already spent about $5 million attacking Hillary Clinton, while they have spent $0 hitting Bernie Sanders, according a Democratic source who tracks media buys.

The only attack ads against Sanders this cycle have come from a super PAC tied to Martin O’Malley, the third candidate in the Democratic race, according to the source.

This week, American Crossroads, the group founded by George W. Bush strategist Karl Rove, released an ad attacking Clinton from the left that echoes many of Sanders’ talking points against her. “Ever wonder how Hillary Clinton can afford so many ads? Chances are, they were paid for with Wall Street cash,” the narrator declares. “Hillary Clinton’s gotten 54 times more money from Wall Street interests than from all of Iowa. Hillary rewarded Wall Street with the $700 billion bailout. And Wall Street made her a multi-millionaire.”

It’s part of a larger effort of Republicans to boost Sanders vis a vis Clinton, in an effort to damage the former secretary of state.

“I think it shows how desperate the Republicans are to prevent me from becoming the nominee,” Clinton said on ABC’s “This Week” Sunday.

“It’s not about whether we’re scared of facing Bernie Sanders or Hillary Clinton,” said American Crossroads spokesperson Ian Prior. “The bottom line is if you’re looking at this race from an objective perspective, Hillary Clinton appears to be most likely nominee.”

The pro-Clinton super PAC Priorities USA responded to the American Crossroads ad with one of their own. “Why are Karl Rove and his republican super PAC coming to Iowa to attack Hillary Clinton?” the Democratic ad’s narrator reads. According to the ad, it’s because they’re afraid of Clinton.

Meanwhile, Sanders responded to the Priorities ad with a fundraising email from campaign manager Jeff Weaver saying, “Goldman Sachs [is] funding Clinton ads in Iowa.” A fraction of Priorities USA donations from the Wall Street mega-bank.

In the last Republican debate, Sanders’ name was mentioned five times. Clinton was mentioned almost 30 times.

In addition to American Crossroads, much of the spending against Clinton has come from Future45, a new group mainly funded by GOP mega-donors Paul Singer and Ken Griffin, who are both billionaire hedge fund managers.

Data provided to NBC News by partner SMG Delta shows less than $5 million spent against Clinton, but their data does not include digital advertising or money from so-called dark money groups. The SMG Delta data shows no spending against Sanders.