Democratic presidential hopeful Bernie Sanders criticizes Goldman Sachs and other banks in this ad for "triggering the financial meltdown." (Bernie Sanders)

DES MOINES — With just four days remaining before the Iowa caucuses, Democratic presidential hopeful Bernie Sanders is planning to launch a new ad that takes aim at Wall Street’s campaign contributions and speaking fees, asserting that Washington is “bought and paid for” by the financial sector.

The 30-second spot, which aides said will debut Friday in Iowa, makes no mention of Democratic presidential candidate Hillary Clinton, but the contrast the Vermont senator is attempting to draw seems pretty clear.

In recent campaign stops, Sanders has chided Clinton, a former senator who represented New York, for accepting more than $600,000 in speaking fees from investment firm Goldman Sachs in the run-up to launching her presidential bid. The money, Sanders has suggested, undercuts Clinton’s ability to push for the kind of Wall Street reforms needed to avoid a repeat of activities that contributed to the 2008 financial crisis — a notion she vigorously disputes.

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Sanders’s new ad singles out Goldman Sachs as viewers see an image of the Manhattan skyline. There’s also a prominent reference to the firm’s recent $5 billion settlement to resolve state and federal investigations into its handling of mortgage-backed securities in the years leading up to the 2008 crisis.

“Our economy works for Wall Street, because it’s rigged by Wall Street, and that’s the problem," the narrator says. “As long as Washington is bought and paid for, we can’t build an economy that works for people.”

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Shortly after news reports surfaced Thursday about Sanders's ad, Clinton's campaign released a statement saying that Sanders "has been making a misleading argument about Hillary Clinton and her toughness when it comes to taking on Wall Street."

"The truth is that Clinton has long stood up to Wall Street," the statement said. "While Senator Sanders pushes proposals scrutinized as unachievable and lacking detail beyond a slogan, Clinton has the strongest, most comprehensive plan to tackle risks on Wall Street and in the financial sector."

The new spot is the second from Sanders this month targeting Wall Street. Clinton aides accused him of going negative in an earlier spot that didn’t mention her by name either but was critical of Democrats who “take millions from big banks and then tell them what to do.”

Aides to Sanders circulated copies of the ad Thursday as the senator was preparing to launch a bus tour through the state in the closing days before Monday’s caucuses, in which Clinton and Sanders are neck and neck, according to recent polls.