But Sanders’s gains in national surveys, like his record-breaking small-donor fund-raising and mammoth rallies, testify to the chord he has touched in the Democratic coalition. His advance on all those fronts suggests that Clinton faces a hard ceiling of support that has frustrated her efforts to close out the race. And, as advisers in both camps acknowledge, Sanders’s national gains means that he arrives in each new state with a bigger and broader base of support than he enjoyed earlier in the contest. To the Sanders camp, Clinton’s support in the race is increasingly following the classic pattern for an incumbent, who often finds most of the voters undecided late in a contest breaking against them and preferring the challenger.

Ironically, the recent candidate who most closely matched Sanders’s late primary gains in national polls was Clinton herself in 2008. In national surveys by the Gallup Organization in late April and early May 2008, Clinton briefly rose to tie Barack Obama, even though he had established a steady lead in delegates.

But Clinton’s situation then wasn’t truly comparable to Sanders’s today, because she started that race with a national lead over Obama in 2007 that approached 30 percentage points in some surveys. Obama then passed her in early spring (after he ran off a dominating series of primary and caucus wins in February) before briefly falling behind again when he was embroiled in a series of controversies (the most damaging of which centered on the disclosure of provocative sermons by Obama’s minister Jeremiah Wright). Obama finally rebounded to a comfortable national lead in the campaign’s last weeks.

Sanders’s situation can be compared more revealingly to the last standing alternative in earlier contests—candidates such as Republicans Ronald Reagan in the 1976 Republican race, and George H.W. Bush in the 1980 GOP contest; as well as Edward M. Kennedy, Gary Hart, Jesse Jackson, and Edmund G. (Jerry) Brown in the 1980, 1984, 1988, and 1992 Democratic contests respectively. And this late in the contest, none of them showed the kind of national strength in polls that Sanders is now displaying.

These candidates began their races in very different positions, according to a Gallup compilation of national primary polls. As the race began, some trailed far behind (Bush and Hart). Kennedy (who had stumbled after an early lead) and Reagan were both initially competitive with the sitting presidents they had challenged (Jimmy Carter and Gerald Ford). Brown and Jackson stood in the middle of the pack.

The common theme in their experience, though, is that after the eventual winner of their race won most of the early contests and established a delegate lead, all six trailed in national surveys. And after falling behind, none of them ever regained the lead in national polls—even though several of them won key states in their race’s final stretch.