President Trump woke up on Wednesday morning, worried not about his attorney general’s upcoming testimony on Capitol Hill but instead about firefighters.

Why? Because the main firefighters union, the International Association of Fire Fighters, endorsed Democrat Joe Biden for president on Monday, after staying out of the 2016 contest. So on Wednesday, Trump proceeded to retweet scores of people who took issue with that endorsement several days ago while responding to a tweet from conservative commentator Dan Bongino.

It was a weird display. Even if all of those Trump retweeted were actually firefighters, the total would amount to a fraction of a fraction of a percent of the union’s membership. This being Twitter, though, there was no indication that those claiming to be firefighters actually were. What’s more, it should hardly be surprising that firefighters who follow Bongino on Twitter might be more favorably inclined to back Trump than Biden.

So why this panicky effort to show that firefighters don’t like Biden? Two reasons: one metaphorical and one more directly political.

The metaphorical reason is fairly obvious. Trump’s political shtick is toughness. He’s the guy who’s going to bring the hammer down on America’s enemies — or, at least, conservative America’s enemies. He deliberately allies himself with the military and with first responders like police and firefighters because it bolsters that rhetoric. To have the group representing a large segment of those first responders endorse a Democrat deals a significant blow to Trump’s effort to claim that he’s speaking on their behalf.

But there’s also that political reason Trump isn’t happy about the firefighters’ endorsement. Trump, not entirely without justification, believes that he is uniquely situated to appeal to the white, working-class voters who make up much of the labor movement. To have a major union back a Democrat deals a blow to that narrative, though, as Trump has noted, it’s not unusual. To have a major union that largely represents working-class white men endorse Biden? That’s more problematic.

Over time, people from households where a union member lives have voted more consistently Republican. We noted after the 2016 election that Trump’s support from union members was the strongest a member of his party had seen since Ronald Reagan. But the margin by which union voters backed Democrats for president and in House races had gotten consistently narrower over the past two decades, priming the pump for Trump’s success.

That shift to the right mirrors a similar shift among whites without college degrees over the past two decades. Whites without a college degree are more heavily supportive of Republicans than people in union households, but both groups have moved to the right.

Trump wants to appeal to union voters. Over the long term, assuming that working-class white men continue to heavily support him, that has become trickier. White men make up a smaller percentage of those represented by unions than they did two decades ago, sliding from a bit under half of the membership to a little more than 40 percent last year.

That, of course, hasn’t stopped the union household vote from becoming more Republican — which may suggest that those who live with union members are driving part of that shift, as well. In 2016, an AFL-CIO exit poll found that household members preferred Hillary Clinton by eight percentage points, while union members preferred her by 19 points. That’s a significant difference.

Trump is clearly worried about those working-class white voters if Biden becomes the Democratic nominee, hence his fusillade on Tuesday morning attempting to undermine the union, rebut the idea that union members back Biden and brush off the idea that first responders didn’t prefer him as a candidate.

There hasn’t been much polling about how union members’ views of Trump have evolved over time. But we can point to one bit of data from those exit polls shown above.

In 2016, union households voted for Democrats by a nine-point margin. In 2018, in an election that was a rebuke of Trump’s presidency, that margin doubled.