They (we’re primarily talking about Pelosi here, but I doubt she was alone in her decision) didn’t have to do it. After all, they started out either ignoring them or belittling them.
But they chose to defend them in response to Trump. Why?
Although Pelosi had earlier pointed out that the Four were only four votes, she had to already have been well aware of how many members of the House were in solidarity with them. It doesn’t seem to have been a majority, as Wednesday’s “impeach Trump” vote failure amply demonstrates. So, what gives?
I’ve heard people say in explanation, “Bernie Sanders changed the party.” No he didn’t. He did well in 2016, much better than expected, but he didn’t change anything except perceptions. What he did do was to move the Overton Window way to the left, and the sky didn’t fall. So it became possible for a candidate to voice leftist beliefs and think that he/she still had a chance of support on a national level.
If Sanders had actually run in the general election of 2016 instead of Hillary, how would he have done? No one knows, although he polled quite well (so did she). It’s too late now for him to win the nomination in 2020, because he’s got fresher leftist competition, but the party is very afraid of losing his enormous bloc of leftist supporters—and that of AOC and the Four—by not defending the Four.
But still, Pelosi’s original calculus about this was different. Not too long ago she didn’t seem to think it was necessary to defend them much, and she seemed to believe that marginalizing them would be good. She must have known (or believed) that siding with the Four might cost the eventual Democratic nominee (and those Democrats running for Congressional seats) dearly in the general. A great many Democratic voters have gotten way more leftist than the general public seems to be ready to tolerate yet, despite decades of the Gramscian march that has helped swell the ranks of the left immeasurably, particularly among young people, so Pelosi knew a candidate must beware of appearing too leftist.
So, what’s going on? One thing I noticed is that a possible turning point occurred when AOC played the race card against Pelosi herself, saying, “the persistent singling out … it got to a point where it was just outright disrespectful … the explicit singling out of newly elected women of color.”
The initial reaction was for many fellow-Democrats to jump to Pelosi’s defense. Why, the very idea of using racism accusations as a tool like that! Shocking! Of course, it’s the favored tool against the right and Democrats have no problem with it when it’s used that way. But against Pelosi? Not done, except by the Four.
Initially it seemed that Pelosi had won that round, too. But it may nevertheless have struck some sort of fear in her (at least, that’s my theory), because it told her that the Four were going to strike back with every weapon at their disposal rather than give in. Pelosi well knows the potency of the race card with the Democratic voters; there is probably no worse charge. So she was wary.
Then, when Trump singled out the Four (without naming them, but he certainly was talking about at least one of them, Ilhan Omar), and did it in terms the left wanted to cast as irredeemably and vilely racist, Pelosi almost had to defend them or risk a racist accusation herself, and risk losing the votes of the racial groups on which the Democrats now depend for victory.
Pelosi may well end up tacking back and forth between supporting the Four and keeping them in check, for the rest of the 2020 election cycle.
[ADDENDUM: In addition, some House members from deep blue districts are probably afraid of being perceived as insufficiently leftist, and of being primaried by younger and more telegenic Justice Democrats. So they may desert Pelosi, as well. Is even Pelosi immune, in her own district, if she decides to run for re-election?]