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The poll surveyed 500 voters who said they either already voted in the Republican primary or were likely to do so. It was conducted Friday and Saturday — after Mr. Romney’s wins in primaries in Arizona and Michigan, and his victory in Wyoming’s caucuses.

Mr. Santorum placed second in each of those.

Late Saturday Mr. Romney also handily won Washington’s caucuses with 38 percent of the vote, well ahead of Mr. Paul’s 25 percent and Mr. Santorum’s 24 percent. Mr. Gingrich trailed with 10 percent.