Donald J. Trump’s approval ratings are the lowest at this stage of a presidency since modern polling began in the mid-20th century. But his party’s base still appears highly supportive, which has discouraged Republicans in Congress from abandoning the president and his legislative agenda.

Even the most recent Russia revelations seemingly haven’t dented support for Mr. Trump among Republicans, who continue to approve of his job performance at very high rates — 82 percent in an ABC News/Washington Post poll released Sunday, for instance.

But numbers like these may mask a decline in support for Mr. Trump among his original party base.

A new working paper by the Emory University political scientists B. Pablo Montagnes, Zachary Peskowitz and Joshua McCrain argues that people who identify as Republican may stop doing so if they disapprove of Trump, creating a false stability in his partisan approval numbers even as the absolute number of people approving him shrinks. Gallup data supports this idea, showing a four-percentage-point decline in G.O.P. identification since the 2016 election that is mirrored in other polling, though to a lesser extent.