Hillary Clinton, meanwhile, now has a 2-to-1 lead over Bernie Sanders in the Democratic race.
Trump has 27% support from likely GOP primary voters nationwide in a survey released Wednesday. He's followed by a second tier that's bunched closely together: Florida Sen. Marco Rubio is at 17%, while Texas Sen. Ted Cruz and neurosurgeon Ben Carson are at 16% each.
Former Florida Gov. Jeb Bush has 5% support, and no other candidates top 3%.
It's a major drop from Carson, who a month ago was neck-and-neck with Trump, whose lead was then just 24% to Carson's 23%.
Among Democrats, Clinton's lead is now 60% to Sanders' 35% -- an improvement for the former secretary of state over an early-November Quinnipiac poll, which showed her leading 53% to 35%. Former Maryland Gov. Martin O'Malley has 2% support.
The poll also shows that general election voters more broadly have shifted their support from the Republican candidates toward Clinton and Sanders as well.
Clinton beats Trump in a head-to-head competition, 47%-41%. She beats Rubio 45%-44%, Cruz 47%-42% and Carson 46%-43%.
Sanders does as well or better in head-to-head match-ups with Republicans, leading Trump 49%-41%, Rubio 44%-43%, Cruz 49%-39% and Carson 47%-41%.
Both of the front-runners have a problem with trustworthiness. Clinton is perceived by 60% of American voters as untrustworthy, compared to 36% who say she is trustworthy. Trump's numbers, meanwhile, are 59% to 35%.
The poll was conducted November 23-30 -- an unusual period for pollsters, since it includes the Thanksgiving holiday, when those surveyed are less likely to answer their phones.
A total of 1,453 American voters were surveyed, with a margin of error of plus or minus 2.6 percentage points. The poll includes 672 Republicans, with a margin of error of plus or minus 3.8 points, and 573 Democrats, with a margin of error of plus or minus 4.1 points.