They noted that Lamb, 33, a handsome (and clean-shaven) military veteran, was straight from central casting and had no extensive political record to contradict stances so moderate and squishy that he could be mistaken, well, for a Republican. They said that Democrats would be hard-pressed to find many more Lambs with which to slaughter the G.O.P. in November.

“When your message is simply I am for new leadership and cleaning up Washington, and you look like you just walked out of an Orvis catalog, you are going to connect with voters on both sides of the aisle,” wrote Salena Zito in The Washington Examiner late last week.

“Lamb never slams Trump,” she observed. “He is not part of the resistance.” He further blunted Republicans’ favorite weapon against Democratic candidates for the House — they’re just would-be pawns of the dreaded Nancy Pelosi! — by running an ad that made clear that he would not support her as his party’s leader in that chamber.

The point that Zito was correctly making was that many of the Democrats who will vie to unseat Republican incumbents in House races in November won’t be able to follow Lamb’s playbook. To get through their party’s primaries, they’ll have to stake out more progressive ground than he did, and adopt a more combative, fiery tone. That could undercut their chances of replicating his success.

Indeed, Democrats’ euphoria over how he fared on Tuesday will give way to sharp internal tensions and sustained quarreling over which sorts of candidates — soft-spoken or bold, centrist or liberal, eclectic or pure — the party would be wisest, from a pragmatic standpoint, to promote. Many Democratic voters want someone less mild and muddled than Lamb.

But if the Pennsylvania results put Democrats in an awkward position, they leave Republicans in an even worse place. What exactly is their best strategy for the midterms?

Not all of their candidates will be leagues better than Saccone, who is no Roy Moore and is himself a military veteran. Sending President Trump into districts that supposedly smile on him isn’t looking like such a hot proposition. The mantra of “tax cuts, tax cuts, tax cuts” is obviously no panacea.

And with each passing week — each passing day — the Trump administration’s turbulence intensifies and the scandals and scandal-ettes pile up. Yes, it’s a long way from now until November, and much about the national mood and the playing field can change. But in that yawning stretch of time, Trump can also render himself and his enablers even less attractive. I have faith.