Donald Trump continues to lead Republican U.S. presidential candidates among likely GOP caucusgoers in Iowa, but retired neurosurgeon Dr. Ben Carson is closing the gap between them, according to a new poll. Trump is first with 23 percent support, and Carson is second with 18 percent support.

The Des Moines Register/Bloomberg Politics survey results found that, although Trump leads Carson by about five percentage points, the latter has the highest favorability rating among the 17 GOP candidates. Just 8 percent of those surveyed held negative views of Carson.

The poll also found support for the other Republican candidates -- all seasoned politicians -- sank to single digits, percentagewise.

"Wow," Kedron Bardwell, an associate professor of political science at Simpson College in Indianola, Iowa, told the Des Moines Register. "This poll will have Republican consultants shaking heads in bewilderment. Not since 1992 has anti-establishment sentiment been this strong." 

Recent polls have found that support for nonestablishment candidates has run in parallel with growing anger at both Democratic and Republican politicians. Three-quarters of those polled in Iowa said they were frustrated with Republicans in Congress, and 21 percent said they were "mad as hell," the Register reported.

Trump has established himself as the Republican front-runner nationwide, as well as in other Iowa surveys.

The real-estate mogul and reality-television personality has reportedly built support among many of the country's conservative communities, notably for his recent affirmation of support for the Bible. Although Carson has not attracted the same level of media attention as has Trump, the retired physician led in the poll among those identified as Christian conservatives.