At the Netroots Nation convention on Friday, Senator Elizabeth Warren delivered a direct challenge to Hillary Clinton and all Democratic presidential campaigns to support legislation that would end the “revolving door” between top government positions and corporate America.

“Anyone who wants to be president should appoint only people who have already demonstrated they are independent,” Warren said to the progressive convention-goers, “who have already demonstrated that they can hold giant banks accountable, who have already demonstrated that they embrace the kind of ambitious economic policies that we need to rebuild opportunity and a strong middle class in this country.”

This is the first time Warren has decided to engage in the presidential election, which activists tried for months to get her to enter. Instead of asking candidates to endorse one of her particular policies on bank reform or student loans, Warren is focusing on the personnel who will implement those policies. It’s a notable choice that dovetails with Warren’s interest in ensuring that executive branch appointments will not tip the scales in favor of Wall Street or private industry, seen most directly in the fight to block Antonio Weiss from the No. 3 position in the Treasury Department. Warren raised the profile of Weiss, a longtime bank executive, enough for him to withdraw his nomination.

“Sure, laws matter. But it also matters who interprets those laws, who enforces those laws,” Warren said. “Think of it this way: How would the world be different today if, when the economic crisis hit, Joe Stiglitz had been secretary of the Treasury and Simon Johnson and Robert Reich had been key economic advisers?”

Hillary Clinton, the frontrunner for the Democratic nomination, has struggled with a perception from the liberal wing of the party that she has close ties to Wall Street, fueled by big bank donations to the Clinton family foundation and speeches she's made at places like Goldman Sachs. While Warren doesn’t mention Clinton by name, the implication is clearly to force her to embrace ending the revolving door.