More damaging to their hopes of stopping Romney, Santorum and Gingrich failed to qualify for the Virginia primary ballot, and Romney appeared in line to capture all 46 delegates there. The former Massachusetts governor also has virtually no competition on his home turf in that state, with 38 delegates, and little in Vermont, with 17 more.
Romney pressed his advantage in other ways, from personal endorsements to a huge disparity in television ads across the country.
One day after winning the support of House Majority Leader Eric Cantor of Virginia and Oklahoma Sen. Tom Coburn, he drew backing from former Attorney General John Ashcroft. Romney has long claimed he is the most electable of all the Republicans in the race, and the endorsements were a fresh sign that the party establishment has begun to rally to his side.
While Santorum recently reported taking in $9 million in campaign donations in February, he was outspent across the board on television.
Romney purchased about $1.5 million in television commercials in Ohio, and Restore our Future, the super PAC that supports him, spent even more, $2.3 million.
Santorum and Red, White and Blue, a super PAC that backs him, countered with about $1 million combined, according to information on file with the Federal Election Commission, a deficit of nearly 4-1.
In Tennessee, where Romney did not purchase television time, Restore Our Future spent more than $600,000 to help him. Santorum paid for a little over $225,000, and Winning our Future, a super PAC that backs Gingrich, nearly $470,000.
In Georgia, where Gingrich acknowledged he must win, the pro-Romney super PAC spent about $1.5 million in hopes of holding the former House speaker below 50 percent of the vote, the threshold needed to maximize his delegate take.
Associated Press writers Kasie Hunt and Steve Peoples in Ohio, Tom Beaumont in Tennessee, Beth Fouhy in New York and Stephen Ohlemacher in Washington contributed to this report.
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