The transformative nature of President Trump's election in 2016 -- by way of a unique coalition of support -- had given Republicans a degree of optimism heading into a challenging midterm election season. Though the president has historically low approval ratings nationally, he is still overwhelmingly popular among the party's base, which includes voters Trump lured away from the Democrats.

But as Democrats discovered over the past few elections with Barack Obama as president, star power doesn't necessarily trickle down. If anything, it fuels and unites the opposition. Now, after losing a congressional district Tuesday that Trump had won by 20 points, the GOP faces a similar reckoning.

Democrat Conor Lamb's apparent defeat of Rick Saccone in a Western Pennsylvania district that combines steel country with the suburbs, comes on the heels of Democrats winning a U.S. Senate seat in Alabama for the first time in a quarter-century. Trump endorsed the Republican candidate in both cases. He visited Pennsylvania twice to tout his agenda and tie it to Saccone, who ran as an ally of the president. Trump's son Don Jr. told voters during a campaign stop earlier this week that "everything [my father] stands for and represents for the future" would be on the ticket.

Republicans say Trump's visit was helpful, and argue that Saccone was a uniquely weak candidate and Lamb as a uniquely strong one. But the results raise questions about the president's utility for his party in the midterms and the transferability of his coalition of support.

“This is a sign that the congressional map is more likely to expand than narrow over the coming months," said GOP strategist Ken Spain, who served as communications director for the National Republican Congressional Committee during the 2010 cycle -- when his party took control of the House.

“In midterm elections, voters are voting ‘yes’ or ‘no’ to the party in charge," Spain said. "While the president will likely be an asset for Republicans in rural and exurban districts, it does not appear – at least at this point in the election cycle – that he is a net positive asset on the 2018 campaign trail.”

Former NRCC Chairman Tom Davis said Republicans shouldn't be counting on Trump to turn out voters anyway. "Obama couldn't turn out Democrats in midterms. Why would it be different for Trump? That's the history of midterm elections. They don't come out in off years," Davis said. "It's a strong, strong base, but the problem is Trump-centric voters aren't coming out for other Republicans."

As midterm campaigns heat up, Republicans will be analyzing where the president can be useful and where he will be a liability. There is some consideration that resistance to Trump among Democrats, though palpable, may already be baked in, so Republican candidates might as well have him visit to drum up support among his voters. But there is some nuance in primaries versus general elections.

“If you're in a primary election, you better be sticking close to Trump wherever you are. But in the general in many of these districts, being attached to Trump doesn't help,” Davis said. “It depends where you are.”

Republican operatives say Trump and Republicans have to carefully strategize about where and how to deploy the president, especially since control of Congress would determine the rest of their shared agenda and serve as a firewall against impeachment. While Republicans debate Trump's usefulness in turning out voters, some strategists say the party needs to more closely align with Trump in order for the transfer factor to pay dividends.

"What he needed to do is get a lot of people to say, 'I'm not necessarily voting Republican, I'm voting for the Trump agenda,’" said GOP strategist John Brabender, who has served as an adviser in many Pennsylvania races. "The real challenge is how to take those conservative Democrats who like the president's agenda and are dissatisfied with both parties and help them understand the president is going to need reinforcement."

Lamb's campaign "was smart enough to figure out that a Democrat running as supportive of the Trump agenda has a better chance of winning in this district. And they got a twofer: a Democratic base fired up, but also those swing Democratic voters choosing the Trump agenda," said Brabender.

Though Lamb didn't run as a Trump ally, he didn't run against him either. Unlike many in his party, Lamb didn't even mention the president much on the campaign trail and instead ran as a conservative Democrat, appealing to union workers and also to suburban voters more inclined to his demeanor than to Trump's. He also embraced Trump's plan to impose tariffs on steel and aluminum imported to the United States, ran a pro-gun message, and said he would not back Democratic Leader Nancy Pelosi. But he also ran against the GOP tax bill and as a supporter of Obamacare.

Lamb's union support was also integral, and demonstrated that a coalition inclined toward Trump in a presidential election isn't easily swayed by him in other races. "It shows what a candidate who is willing to stand together with workers and embrace our agenda, what kind of activism that can motivate," said Josh Goldstein, spokesman for the AFL-CIO. "There are a lot of races coming up in these midterms where there will be candidates who are willing to or able to inspire working people."

While Republicans argue Lamb's candidacy was singular enough to win a district like PA-18, Democrats point to ratings that show over 100 congressional districts as more competitive than PA-18 was on paper. And they note districts like the 12th in southern Illinois, where veteran and prosecutor Brendan Kelly is challenging Republican Mike Bost in an area Trump won by 15 points. Democrats have also touted a flood of military veteran recruitments. And they point to the Pennsylvania race as a test case for whether the GOP message on tax cuts and the economy is working.

This notion also raises the question of whether Trump can be an effective messenger on the agenda items Republicans wish he would talk about or concentrate on.

"Democrats want 2018 to be a race about the president and his personality. Republicans want to run on the issues like the tax bill and the improving economy. But the president won in 2016 in large part because he ran on personality," said Spain. "As a result you have diverging messaging efforts playing out on the GOP side."