Hillary Clinton waves to voters following a campaign stop at the Hawkeye Labor Council AFL-CIO Labor Day picnic in Cedar Rapids, Iowa, Sept. 7, 2015. The share of Democrats is at its lowest level in decades, a new poll states. Reuters

The share of Democratic voters in the United States is at its smallest level in nearly 30 years, according to a Gallup poll published Monday. In 2015, 42 percent of voters identified as independents, 29 percent as Democrats and 26 percent as Republicans, the poll found.

Democrats haven't had such little support since 1988, when the polling institution began interviewing people on the phone about their political affiliations. But in data collected in person from 1951 to 1987, a minimum of at least 37 percent of Americans always identified as Democrats, Politico reported.

For independents, there was a slight drop in followers compared with the 43 percent who identified as such in 2014. That year, 30 percent of Americans identified as Democrats. Republicans marked their lowest level of support in 2013, when 25 percent of Americans identified as GOP voters. The poll surveyed 12,137 Americans in landline and cell phone interviews throughout 2015. It had a margin of error of plus or minus 1 percentage point.

Any drop in support could spell trouble for Democrats, who are hoping to keep the White House after eight years of Democratic President Barack Obama. In a poll released over the weekend, about 20 percent of likely Democratic voters said they were considering voting for Republican presidential candidate Donald Trump in a general election. On the other hand, the figures released by Mercury Analytics also showed 14 percent of Republicans would vote for Democratic front-runner Hillary Clinton. The company surveyed about 916 likely voters last week and the poll has a margin of error of plus or minus 3.5 percent, the Hill reported.

In general, Democrats tend to do well with blacks, Asians, Hispanics, well-educated adults and millennials. Republicans are the favorite among white men, those with less education, evangelical Protestants and members of the Silent Generation (those now over 70), according to Pew Research Center data.