News analysis

In a result that many operatives considered unthinkable last year, Democrat Conor Lamb appears to have edged out Republican Rick Saccone to win a district President Trump carried by nearly 20 points in 2016. If Lamb's razor-thin lead survives a possible recount, his win would move Democrats one seat closer to the 24 they need to win the majority, and could spike Republicans' already-high anxiety about losing control of the House.

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Taken alone, it's easy to dismiss Pennsylvania's 18th Congressional District as just one race out of 435. But taken together with five other deeply red districts where Democrats have over-performed in the past year, the results fit a disturbing pattern for Republicans. Here are five takeaways from Tuesday's election:

1) President Donald Trump's low approval rating probably wouldn't have been enough to give Democrats an even shot to win the district on its own, but it was a necessary precondition. Intensity was highest in the places where support for Trump was lowest: Turnout was at 67 percent of 2016 levels in Allegheny County, where Lamb won, compared to 60 percent in Westmoreland County, where Saccone led handily.

2) The outcome had more to do with Lamb's strengths than Saccone's weaknesses. Saccone may have been an unexciting state legislator, but he never made any major missteps. Lamb simply turned out to be Democrats' dream candidate because he fit the district: A young former Marine and prosecutor, Lamb charismatically appealed to union households, older voters, Catholics and other key constituencies.

3) It may sound counterintuitive, but House Democrats were wise to maintain a hands-off approach to the race. They spent only a fraction of the money House Republicans spent on ads, but this allowed Lamb to run his own race and credibly make the case that he wasn't beholden to the national party. Lamb broke with his party on energy, tariffs and new gun measures — and he pledged not to vote for Nancy Pelosi as speaker.

4) Republicans couldn't find a message that moved numbers, even in a heavily pro-Trump district where Trump campaigned for Saccone in person twice. At first, Republican outside groups sought to call Lamb a "sheep" in Pelosi's "flock." Then they championed the tax cut bill and portrayed Saccone as the only candidate who would protect voters' extra take-home pay. Finally, they sought to attack Lamb's record as prosecutor. Nothing worked.

5) According to the Cook Political Report's Partisan Voter Index, there are 118 GOP-held House districts that are less Republican than this one. Tuesday's result doesn't mean all of those seats will be in play in November, but a few more GOP incumbents may contemplate retiring. And Republicans could face a key special election test on Aug. 7 in Ohio's 12th District — a seat 4 percentage points less Republican than the one they apparently lost on Tuesday.

David Wasserman, House editor for The Cook Political Report, is an NBC News contributor and senior analyst with the NBC Election Unit.