Ms. Warren has also raced, for better or worse, to stake out firm positions on several divisive issues: She has, for instance, called to end the Senate filibuster, to eliminate the Electoral College and to impeach Mr. Trump, edging farther than Mr. Sanders has.

But for most of this primary season, Mr. Sanders, with his fervent base, a presidential campaign under his belt and a network of reliable, small-dollar donors, has loomed over his liberal rival. Now, as the race for the Democratic nomination heads into the crucial summer season, Ms. Warren has managed to break out of that shadow, defying those in the party who doubted before the race began that she could.

By running a campaign heavy on both policy and biographical details, she has wrested some high-profile liberal supporters away from Mr. Sanders, and in some polls, has shown signs of ticking upward.

Mr. Sanders himself has even adopted some of Ms. Warren’s signature campaign tactics: He has reintroduced grass-roots fund-raisers, for instance, to say nothing of his decision to start posing for selfies.

But while Ms. Warren has gained ground, she has not yet cast a shadow of her own.

Mr. Sanders still holds a large advantage in the polls — a point his advisers eagerly highlight — and his supporters say he remains the clear progressive standard-bearer among the larger electorate. If the ideas that Ms. Warren has introduced in this presidential cycle are popular, aides argue, many of them can be tied to ones Mr. Sanders introduced during his 2016 campaign, when he thrust issues like “Medicare for all” and tuition-free public college into the national consciousness.

“A lot of these policies that we’ve heard articulated from other candidates are policies that have been core to Bernie Sanders and the case that he’s been making for a very long time,” said Faiz Shakir, Mr. Sanders’s campaign manager.