President Trump, shown with House Majority Whip Steve Scalise (R-La.), speaks during a meeting at the White House on March 7. (Jabin Botsford/The Washington Post)

One of the arguments Donald Trump used on the campaign trail to suggest that his election was inevitable was that frustrated Democrats would cross party lines to support his candidacy. It’s clear that this actually happened, and perhaps happened enough to make the difference in the three states he narrowly won to clinch the electoral college majority and the presidency.

But new analysis from the Pew Research Center suggests that flipping parties between late 2015 and now was fairly uncommon — and that the most volatile group of voters should sound an alarm for Republicans over the long term.

Overall, about 20 percent of those in each party tracked by Pew wavered on their partisan support, with about half of that 20 percent eventually returning to their original party. About 10 percent of Democrats and Republicans, in other words, left their party and joined the opposition.

When you look at the data by age, though, the patterns are interesting. Among Democrats, those ages 50 to 64 were most likely to switch to the Republican Party — a group that clearly overlaps, to some extent, with those to whom Trump felt he held an appeal.

Look at those Republican numbers, though.

Nearly half of Republicans younger than 30 bailed on the party at some point, with 23 percent of that group flipping to the Democrats. The entire point of this research, of course, is that partisan identity isn’t entirely static, and younger voters may still be in the mode of determining where their political loyalties lie. If that’s the case, though, that phenomenon is restricted to the GOP; young Democrats were less likely to switch party identity than older Democrats.

The problem for Republicans is that those young voters may stay away from the GOP. Studies have shown that partisan identity is formed early on, with partisanship tending to correlate to the popularity of the president in office. As FiveThirtyEight noted in 2014, the most fervent Republican voters are those who were 18 at the outset of the Eisenhower and Reagan presidencies; the most Democratic were those who turned 18 as George W. Bush was mired in the Iraq War.

That’s the alarm bell that should be ringing at Republican Party headquarters right now: a historically unpopular president and young voters who are balking at the party. The period considered by Pew encompasses Trump’s campaign, of course, and just as those older Democrats who defected probably did so in part because of Trump, it’s fair to assume that some number of the younger Republicans who did so were driven by his candidacy, as well.