From Gallup:
…the percentage of U.S. adults identifying as Democrats is now at the lowest point in the past 27 years, down from the prior low of 30% in 2014. Gallup’s shift from in-person to telephone interviewing in 1988 complicates the ability to directly compare party identification data collected between the two methods. However, Gallup data from 1951-1987 collected in person never found a yearly average Democratic identification less than 37%, making it safe to conclude that the current 29% is also the low point in Gallup polling history.
The percentage of Republicans is now just one point above its recent low of 25% in 2013. Before 1988, there were several years when the average percentage of Republican identifiers in Gallup polls was lower than 25%.
However when the political preferences of independent voters are factored into the data, things look slightly better:
Last year, in addition to the 29% of Americans who identified as Democrats, another 16% said they were independents but leaned toward the Democratic Party, for a combined total of 45% Democrats and Democratic leaners among the U.S. population. Likewise, 26% of Americans identified as Republicans and an additional 16% identified as independents but leaned toward the Republican Party, for a combined total of 42% Republicans and Republican leaners.