One day after Hillary Clinton wooed deep-pocketed Bay Area donors by unveiling a new policy platform that was a veritable love letter to Silicon Valley—complete with student loan giveaways to start-up founders and easier access to foreign work visas—Elizabeth Warren railed against the same tech giants as cold-hearted monopolists, calling into question whether the Massachusetts senator’s name makes sense atop Clinton’s V.P. shortlist.

“Anyone who loves markets knows that for markets to work, there has to be competition,“ Warren said in the keynote speech for New America’s Open Markets event on Wednesday. “But today, in America, competition is dying.” The popular Democratic senator’s criticism wasn’t limited to Wall Street, either. Warning that “big guys can lock out smaller guys and newer guys,” Warren called out the technology sector specifically for this offense. “Google, Apple, and Amazon provide platforms that lots of other companies depend on for survival. But Google, Apple, and Amazon also, in many cases, compete with those same small companies, so that the platform can become a tool to snuff out competition.”

By singling out these three companies in particular, Warren took a simultaneous swing at some of Clinton’s most powerful allies in tech. Eric Schmidt, the executive chairman of Alphabet, Google’s parent company, was among a number of Silicon Valley heavyweights that threw their support behind Clinton in recent weeks. Jeff Bezos, the C.E.O. of Amazon has also been a vocal critic of Donald Trump and Peter Thiel, a Trump delegate who is one of the few tech titans not backing Clinton. Similarly, Apple C.E.O. Tim Cook has been at odds with the presumptive G.O.P. nominee for months over encryption (Trump has called to boycott the $521 billion company) and Apple’s manufacturing plants, many of which are based in China. These are, in other words, powerful allies that Clinton wants to keep around. But Warren struggled Wednesday to toe the party line, highlighting the stark differences in their ideological approaches to corporate America. “Google, Apple, and Amazon have created disruptive technology that changed the world, and every day they deliver enormously valuable products. They deserve to be highly profitable and successful,“ Warren said. “But the opportunity to compete must remain open for new entrants and smaller competitors that want their chance to change the world again.”

This line of criticism is nothing new for the former law professor. Much like Clinton’s hanger-on Democratic rival, Bernie Sanders, Warren has built a career on her reputation as a champion of the middle class and a foe of the 1 percent. And when it comes to income distribution, the landscape in Silicon Valley is pretty similar to that on Wall Street. But unlike Warren, Clinton has friendships she needs to maintain with the economic elites who have bankrolled her career.

Since endorsing Clinton, Warren has proven to be an effective attack dog and risen to the top of the pile of potential V.P. picks for the former secretary of state. But a good attack dog needs a good leash, and the senator has shown few signs she’s willing to set her deeply-held beliefs aside. In conjunction with Virginia Senator Tim Kaine facing scrutiny over his questionable finances, Clinton’s shortlist is suddenly looking a little weak. Castro for vice president?