President Trump and Senator Elizabeth Warren make odd antitrust bedfellows. Ms. Warren, the Massachusetts Democrat, says megadeals like Aetna’s $77 billion sale to CVS could kill competition. She also backs the Justice Department’s fight against an AT&T-Time Warner merger and has concerns about past merger remedies. That puts her in the same camp as the president.

She laid out her views on deal making in a speech on Wednesday. AT&T’s $85 billion acquisition of Time Warner, she said, would mean higher prices, fewer choices and worse service for consumers. That echoes Mr. Trump’s antitrust chief, Makan Delrahim, and the Justice Department’s November lawsuit to block the deal, which asserted that the merger would leave millions of television viewers paying more and would slow innovations like video streaming.

Ms. Warren also said past concession agreements that allowed certain mergers to go forward had been “epic failures.” That almost feels like a hat tip to Mr. Delrahim, who in a speech last month said past behavioral remedies to address antitrust concerns had not worked out as planned and had been difficult to enforce. He cited Comcast’s takeover of NBCUniversal as an example, adding extra execution risk to vertical mergers in which deal partners don’t directly compete.

His tough stance surprised the corporate sector, which had expected easier deal reviews under the Trump administration. It’s also rare for vertical mergers, like AT&T’s acquisition of Time Warner, to face court challenges since they are usually approved with conditions.