A widely held tenet of the current conventional wisdom is that while President Trump might not be popular overall, he has a high floor on his support. Trump's sizable and enthusiastic base -- perhaps 35 to 40 percent of the country -- won't abandon him any time soon, the theory goes, and they don't necessarily care about some of the controversies that the "mainstream media" treats as game-changing developments.

It's an entirely reasonable theory. We live in a highly partisan epoch, and voters are usually loyal to politicians from their party. Trump endured a lot of turbulence in the general election but stuck it out to win the Electoral College. The media doesn't always guess right about which stories will resonate with voters.

But the theory isn't supported by the evidence. To the contrary, Trump's base seems to be eroding. There's been a considerable decline in the number of Americans who strongly approve of Trump, from a peak of around 30 percent in February to just 21 or 22 percent of the electorate now. (The decline in Trump's strong approval ratings is larger than the overall decline in his approval ratings, in fact.) Far from having unconditional love from his base, Trump has already lost almost a third of his strong support. And voters who strongly disapprove of Trump outnumber those who strongly approve of him by about a 2-to-1 ratio, which could presage an "enthusiasm gap" that works against Trump at the midterms. The data suggests, in particular, that the GOP's initial attempt (and failure) in March to pass its unpopular health care bill may have cost Trump with his core supporters.