Candidates with a case for inevitability — the ones who started as big favorites and won the nomination without a long fight, like Al Gore and George W. Bush in 2000 and Bob Dole in 1996 — all held at least 50 percent of the vote in early polls, and led their opposition by enormous margins. The record of candidates with similar standing to Mrs. Clinton, like Gerald Ford in 1976 or Ted Kennedy in 1980, is not at all perfect. Kennedy lost, and Ford faced a protracted contest.

Flash-forward to 2015. No candidate, excluding incumbent presidents, has ever fared so well in the early primary polls as Mrs. Clinton. She holds about 60 percent of the vote of Democratic voters, a tally dwarfing the 40 percent she held this time in the last election cycle.

If anything, in the 2008 cycle the national polls overstated Mrs. Clinton’s strength. She trailed in Iowa polls from the very start. She led in New Hampshire and South Carolina only by single digits, making it easy to imagine how the winner of Iowa could gain momentum and go on to defeat her in following contests.

Her vulnerabilities were obvious. Her vote to authorize the war in Iraq was a serious liability; so were reservations about another Clinton in the White House. Unlike Mr. Bush, Mr. Gore or Mr. Dole, Mrs. Clinton faced two top-tier challengers, the former vice-presidential nominee John Edwards and Mr. Obama, a rising star thanks in part to his speech at the 2004 Democratic National Convention.

Mr. Obama had already declared his candidacy by this time in 2007. He had surged to 25 percent in the polls. Enthusiastic crowds showed up to early rallies in Austin, Tex., and Oakland, Calif. He matched Mrs. Clinton in fund-raising in the first quarter, demonstrating strong support in the so-called invisible primary — the behind-the-scenes competition for the resources and credibility necessary to win the nomination.