The Texas Railroad Commission basically requires that there be at least one white Pullman employee on each sleeper car. Pullman had been cutting back on white workers because they could pay black people less, so the groups were pitted against one another, but they won political favor in TX.
So the idea here is that the challenged law could be invalidated under the TX statute that empowers the railroad commission-- can it make a law requiring that the white workers have jobs?
Isn't it a duty of the court to articulate federal rights, and aren't the federal courts the primary guardians of constitutional rights? But here we have state law in federal courts, and the final interpreter of state law should be the state supreme court. So maybe we want to hint to the state court that they should construe their statute in order to avoid the constitutional question. That's one way of avoiding needless friction with the state courts-- it's a little invasive, but needfully so, because the alternative might be to strike down the state statute.
So this doctrine (typical of Frankfurter) is deferential to federalism, but it's not terribly efficient. Frankfurter thinks that this avoidance of jurisdiction is a good thing (see quote on the top of p. 648). He even talks about efficiency, but he's talking about efficiency for the federal judges, not for the plaintiffs. And indeed, perhaps thanks to this procedural obstacle, this case never went anywhere after this absention.
Still, maybe (and recall Frankfurter's dissent in Monroe v. Pape) the federal courts should be a last resort, and it's better to try and resolve problems within the state.
OK, so what if the state court resolves the constitutional issue anyway? That's England: if you want to reserve your federal issue for federal court, make it explicit.
And note that this is basically a technique to get an advisory opinion from the state court: a ruling that can then be plugged into the federal case. So if you're against advisory opinions (and indeed, Federal courts could not do this), you might think this isn't the greatest idea.
As a footnote, even when there's a statutory certification procedure, so that a federal court can directly ask the state supreme court a question (WI has this), it's not necessarily convenient.
Anyway, this is a doctrine that prefers getting authoritative opinions about state law to efficiency.
So there is a federal statute (§ 2283) against enjoining state proceedings, but we saw in Mitchum that this can be overcome. So then there was also Dombrowski v. Pfister, which appeared to say it's possible to enjoin prosecution when the state is trying to enforce overly broad laws that impugn 1A rights.
But the difference there was that there the prosecutor was more or less harrassing the defendants-- bad faith prosecution. And the state court (therefore) wasn't really in a position to cure the ill-- the laws themselves weren't the problem, it was the prosecutor. Here there's just a single good-faith prosecution, and there's no irreparable injury that goes beyond what can be dealt with at the state level: the state can cure it within the context of the proceeding at hand.
And note that this is the end of the Warren court, and the start of the Burger court. One of the issues in Nixon's 1968 election was getting the crazy people off the court and putting on some more stable entities. But even Brennan and Marshall (holdovers from the Warren era) concur on this, so it can't be too much of an ideological departure.
Harris has an injury in fact for standing purposes, but he doesn't have an irreparable injury in fact for injunction purposes, so it's OK to let his case proceed in state court. We don't want every criminal prosecution to give rise to a § 1983 claim, basically. But do we want an exception to Younger for facial challenges to statutes under 1A?
That's one of the criticisms of Younger, but note the discussion of how injunctions couldn't really cure the chilling effect anyway (p. 659). So go to state court, and if they let you down, you've got habeas (§ 2254) or appeal in state court and then to SCOTUS. You can't use § 1983 as a strategy to invoke federal jurisdiction to review the state court decision (this is the Rooker-Feldman doctrine).
So if we file our federal claim before the state prosecution is underway, can we avoid Younger? Yes, but we don't avoid the problem of standing. Compare Steffel with the other would-be defendants in Younger: he's got a much more concrete and particularized issue, so he has standing, whereas they did not.
So there's a window: on one side we have standing problems, and on the other, we've got Younger. That's what Hicks v. Miranda is about-- can there be a race to the courtroom. And we worry about this exploit: if the state wants to get out of being sued in federal court, can they just start prosecuting you? If the federal court hasn't gone far enough, it appears like you would be Younger-ed out of there. So there's not really a race to the courhouse-- the federal court tends to defer (there's a lot of caselaw about how much federal proceeding is enough to bar Younger abstention: were there "proceedings of substance on the merits?").