There is perhaps no more abstract and imaginary idea in American politics than "public opinion." It is a nebulous concept that we define and measure in multiple ways, all of them imperfect. What does it mean for the president, for instance, to have the support of the public? It affects his decisions and those of other political actors only if they all decide to believe it matters. There's no national plebiscite on whether President Trump should send more troops to Afghanistan or whether Republicans in Congress should resist his version of tax reform in favor of theirs, but we all agree that the more citizens who tell pollsters they're happy with his performance, the more political actors will line up behind him when making their own decisions.

Trump has been unpopular from the start, but now, five months into his presidency, something dangerous may be happening: He could be losing support from Republican voters.

This is particularly important for Trump, because he starts off with essentially no support from Democrats. It seems like long ago, but there was a time when we expected a president to hold the good will of a healthy chunk of the other party's voters. Over his term, Richard Nixon averaged 34 percent approval from Democrats in Gallup polls; Jimmy Carter got 30 percent from Republicans, Ronald Reagan got 31 percent from Democrats, and even Bill Clinton averaged 27 percent approval from GOP voters. It was during George W. Bush's term (after his post-9/11 approval glow faded) that things got extraordinarily polarized. For the last two years of his term in office, Bush never cracked double digits among Democrats. Barack Obama didn't do much better; his approval among Republicans was between 10 and 15 percent for most of his tenure.

That near-unanimous opposition from the other side means that a contemporary president has to hold the support of nearly all of his own party's voters in order to crack that 50 percent margin that convinces everyone he's popular and should therefore be heeded. But when the president's numbers are in the 40s — or even the 30s — people stop being afraid of him. And that can cause all kinds of trouble.

That's where President Trump is now. A CBS News poll released Tuesday shows his approval at 36 percent, the lowest he has been in that poll since taking office. The Pew Research Center puts him at 39 percent. Gallup shows him at 37. An AP/NORC poll says the number is 35 percent.

It's not news that Democrats are united in their dislike of Trump. But there are glimmers of Republican dissatisfaction, too — the CBS poll shows him with the approval of only 72 percent of Republicans; Pew puts the number at 81, while the AP says 75. As early as it is in his administration, if he can't boost that approval up, one group of people will start looking at him very differently: members of his own party in Congress. And that could be trouble.

Trump famously said during the 2016 primaries that he could shoot somebody in the middle of Fifth Avenue and not lose any support. And he seemed to be right, when you extend the prediction to things like "pick a fight with a Gold Star family," "make racist attacks on the judge overseeing the lawsuit regarding my 'university' scam," and "get caught on tape bragging about my ability to sexually assault women with impunity." But the truth is that the voters he was really referring to weren't his entire base.

They were the ones for whom voting for Trump was a statement not of policy preference but of identity, a way of striking a blow against urban elites and the forces of "political correctness" that kept them from saying what they really thought about foreigners and minorities. There were certainly quite a lot of them, but there were also many Republicans for whom voting for Trump was simply a way of getting conservative policy outcomes. They voted for him not because he was a boorish, bigoted ignoramus, but in spite of that fact. And it's those Republican voters whose support he could lose.

There are a number of ways it could happen. One is if he simply fails to deliver — as he has, for the most part, so far. Apart from some regulatory rollback, thus far Trump's administration is remarkably thin on achievements, with not a single significant piece of legislation signed. He did appoint a hard-right Supreme Court justice (which he always describes as though it were some kind of extraordinary achievement). That meant a lot to Republican voters, but there's only so long you can ride in that parade float.

The Russia scandal could also alienate Trump even from many Republicans, who probably can't help thinking, "Wouldn't things be a lot better if Mike Pence were president?" Any Democrat can tell you how tiring it can be to defend a president being consumed by scandal — but at least in 1998 Democrats could argue that Bill Clinton's misdeeds were personal and he was doing an excellent job otherwise. If Trump is lashing out madly on Twitter against the investigation and not getting much of substance done, Republican voters may grow more and more disillusioned.

Should the polls begin to show that, Republican officeholders may decide that the best way to ensure their own survival is to put some distance between themselves and the president. That could mean criticizing him publicly and declining to support his policy initiatives, which would send a signal to GOP voters that you don't have to support Trump in order to consider yourself a Republican in good standing, accelerating a cycle that could spin Trump's approval ratings even lower.

That's mostly hypothetical at this point — Trump's ratings are bad, but other presidents have seen worse (even if not so early in their presidencies). Some policy achievements could turn things around. The Russia investigation could come to nothing. We're certainly a long way from Republicans in Congress deciding that impeachment is the best of the bad options available when it comes to dealing with their president. But the outlook isn't great.