Retired neurosurgeon Ben Carson can no longer call himself a "front-runner" in the race for the 2016 GOP nomination, and Ted Cruz and Marco Rubio are making inroads in the crowded field of White House hopefuls, a new national poll revealed.

Carson -- the soft-spoken physician who had surprisingly topped surveys in recent months but whose campaign has since been hurt by questions about his accounts of a violent childhood -- dropped 7 percentage points in the Quinnipiac University poll. That means the Michigan native now trails real-estate tycoon Donald Trump by double digits, ABC News noted.

And while Trump was able to reclaim his status as sole GOP front-runner, the survey's results also proved encouraging for Cruz and Rubio, who are picking up steam as the former "Apprentice" star's main challengers.

The millionaire businessman now scores the support of 27 percent of Republicans, while the Florida senator has the backing of 17 percent, Quinnipiac detailed. His Texas colleague, meanwhile, ties with Carson at 16 percent.

"Dr. Ben Carson, moving to center stage just one month ago, now needs some CPR," quipped Tim Malloy, the assistant director of the poll. "The Doctor sinks. The Donald soars. The GOP, 11 months from the election, has to be thinking, 'This could be the guy.'"

Nevertheless, 26 percent of GOP voters say they "would definitely not support" Trump, pointing to high negatives for the outspoken New York mogul, according to the poll, which was conducted among 672 Republicans between Nov. 23 and Nov. 30 and has a margin of error of plus or minus 3.8 percentage points.

Rubio, the son of Cuban immigrants, meanwhile, earns the highest favorability marks of all GOP candidates, Real Clear Politics pointed out. Sixty-six percent of Republican voters view the Florida senator favorably, with only 8 percent having an unfavorable impression.

Cruz, a Tea Party favorite, meanwhile, has a favorability versus unfavorability rating of 65 percent to 9 percent, the publication noted.