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Mr. Santorum added to his previous four victories with a win in North Dakota’s caucuses and in primaries in Tennessee and Oklahoma.

But Mr. Romney won Tuesday with his six victories: caucuses in Alaska and Idaho and primaries in Massachusetts, Vermont, Virginia and Ohio.

Democrats argued Mr. Romney should have sealed the nomination earlier against what analysts say is a weak field.

Jim Manley, a former aide to Sen. Edward M. Kennedy who has been watching Mr. Romney since his failed bid to unseat the now late Mr. Kennedy in 1994, said the Republican candidate has a fundamental difficulty in reaching voters.

“The reason people aren’t willing to accept him now is the same reason why he lost against Sen. Kennedy in ‘94,” Mr. Manley said. “When it comes right down to it, people don’t have the faintest idea what he stands for. He has no backbone, no spine. He was for Romneycare but is against Obamacare. Give me a break.”

But he may be suffering from moving goal posts.

The Times analysis found that while some previous Republican nominees had sewn up the race by this point, Mr. Romney is slightly ahead of where Mr. McCain was in 2008.

This article is based in part on wire service reports.