Sanders’s central theme is that the rich and powerful have distorted our political and economic systems to favor their own selfish interests. He blasts Clinton not only for the Goldman speeches but also for mining Wall Street for campaign cash. My assumption is that Sanders, should he fall short of the nomination, will give Clinton his full-throated support. But will his most ardent supporters follow?

As Clinton’s running mate, Warren could erase this potential weakness with the Democratic base. She has spent her Senate career becoming known as the scourge of Wall Street. No political figure is more closely identified with efforts to curb the excesses of the financial system.

Warren would also help address another potential vulnerability. If the general-election matchup is Trump vs. Clinton — and that seems increasingly likely — it is becoming clear that on the question of U.S. military involvement around the world, Trump will position himself to the left of Clinton.

The foreign policy speech that Trump delivered Wednesday was, for the most part, vague and anodyne. His overarching theme is “America first,” he said. To the extent the phrase means anything, it seems to promise that a President Trump would be extremely reluctant to deploy U.S. combat forces in any sort of “world’s policeman” role. Trump has even questioned the viability of NATO in its present form.