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As of this writing Lamb is a few hundred votes ahead with a few thousand ballots to be counted. Still razor-thin, but guesstimates are that he will eek out a win. Whatever the outcome, the victor cannot proclaim a mandate.

But as a canary in a coal mine (cough, cough) this cannot portend well for the GOP nationally. This is Georgia north or the Mason-Dixon line, except the candidate was not an accused pedophile. Trump won by nearly 20 points in 2016. Even if Lamb loses, there can be small comfort to GOP congressional Reps from less red districts.

Also worth noting that this is a special election for a district that no longer exists, if the court-drawn districts hold up. Unless there is a reversal between now and November, Pennsylvania will used the court-drawn district lines, which will further erode the GOP's effectiveness.

A nice little graphic from 538 illustrating the biggest swings in these special elections:


I think this is the best illustration of how a potential blue wave could work, or fall short (I think it is still to early to claim it WILL happen). If the national mood favors Democrats by 17 points that is enough to swing a bunch of seats, or it could also put them just outside the realm of winning.

As you noted the 18th won't exist come November, all things being equal, and we are left to a world where it is actually quite probable that both Lamb and Saccone could be in Washington together representing the 17th and 14th respectively. One of the silliest parts of this is that given how narrow the margins are and the possibility of a recount both individuals may have to declare they are running for those other seats before they know who the winner here is as the deadline to file is the 20th.