"If you win New York and California, arguably Florida and Michigan (where disputed primaries were held), Texas and Ohio and then Pennsylvania — she has the argument, 'I won all the big states, the major contests where we were campaign toe to toe,' " said Daron Shaw, a strategist in the Bush campaigns in 2000 and 2004 who now teaches at the University of Texas at Austin.
The Obama camp was countering with what campaign manager David Plouffe called "the cold, hard reality of the math" that has given the Illinois senator a lead in convention delegates.
Democratic rules that award delegates proportionally rather than winner-take-all make it difficult for Clinton to overcome his edge.
A total of 370 delegates up for grabs in Tuesday's contests. Clinton picked up at least 115 delegates in Ohio, Rhode Island, Vermont and Texas, while Obama picked up at least 88. Nearly 170 delegates were still to be awarded, including 154 in Texas.
Obama had 1,477 delegates, including separately chosen party and elected officials known as superdelegates, according to the Associated Press count. He picked up three superdelegate endorsements Tuesday,
Clinton had 1,391 delegates. It will take 2,025 delegates to secure the Democratic nomination at the party's national convention in late August in Denver.
With that, the loyalty of more than 80% of the delegates to the August convention are settled.
Even after her victories Tuesday, "the truth is that Hillary Clinton's still got an uphill struggle for the nomination," says Democratic pollster Mark Mellman, a top adviser to 2004 Democratic nominee John Kerry who isn't affiliated with a campaign this year.
"She has to make up Obama's delegate lead. Is that possible? Sure. Is it likely? Not very."
"She needs delegates," says Charlie Cook, editor and publisher of the non-partisan Cook Political Report.
An extended calendar could make critical states whose contests were expected to be meaningless because they fell so late in the primary season: Wyoming caucuses on Saturday and the Mississippi primary next Tuesday, followed by a six-week hiatus until the Pennsylvania primary.
The final contest is caucuses on June 7 in Puerto Rico. At stake: 55 delegates.