Rick Santorum, who announced Tuesday he was suspending his campaign, was the choice of 24 percent of Republicans in the poll completed before he made his announcement, the Princeton, N.J., pollster said.
Romney has held the edge over Santorum since late February and has been the leader for all but two brief periods this year.
Former House Speaker Newt Gingrich tied with U.S. Rep. Ron Paul of Texas at 10 percent.
Even though Romney has been among the leaders throughout the GOP nominating process, Republicans' lack of enthusiasm for him has shown several times when he either shared or lost the lead, Gallup said.
Gallup said it was shifting its tracking to measure general election preferences between President Obama and Romney, with the first results to be reported Monday.
Romney's 42 percent in Gallup's final 2012 nomination preference poll is among the lowest Gallup has measured for a party nominee in its final poll since 1972, the pollster said.
Results are based on nationwide telephone interviews conducted April 4-9 with 1,149 Republicans and Republican-leaning independents who are registered to vote. The margin of error is 4 percentage points.
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