Aside from the unexpected decisive victory of once and future President Donald Trump, one of the most surprising results of Tuesday’s election was this: Remarkable victory for Republican Dave McCormick three-term U.S. Senator Bob Casey Jr. (D., Pa.).

Casey did not accept the raceand once more ballots are counted, it’s possible that the narrow lead could trigger Pennsylvania’s automatic recount process. But the Associated Press called the race for McCormick on Thursday, with Casey facing major hurdles in overturning the result.

” READ MORE: In a surprising upset, Republican Dave McCormick unseated longtime U.S. Sen. Bob Casey, according to the AP.

Assuming the AP’s call is valid, the result would be an earthquake in Pennsylvania politics. A moderate who often outperformed his party’s leading candidates, Casey had won six statewide elections in Pennsylvania and his father was a prominent governor.

Christopher Nicholas, a veteran Pennsylvania Republican strategist, said Casey’s popularity has been falling for months and McCormick, a former hedge fund manager with plenty of money behind him, is well positioned to take advantage.

“You always look at the incumbent’s poll numbers and the opponent’s money,” said Nicholas, who worked on the late Sen. Arlen Specter’s campaigns. “Casey’s numbers continued to drop. He was a typical leading from behind type of guy. “And when poll numbers drop below 50, it’s always difficult for an incumbent to move back up.”

All three of Casey’s wins in Senate races came during Democratic strong years: the 2006 midterm elections, dominated by the Iraq War and the growing popularity of then-President George W. Bush; former President Barack Obama’s re-election in 2012; and the “Trump pushback cycle,” as Nicholas put it, in 2018.

“Casey, unlike the other three Senate races, didn’t have the wind at his back,” Nicholas said. “He never had to worry about a rough October.”

So how did McCormick pull off the win? It came down to Trump, messaging and money.

A complicated case of coat tail

In 2012, Casey outpaced then-President Barack Obama by 1.7 percentage points among Pennsylvania voters, and his margin of victory over GOP Senate candidate Tom Smith was 3.7 percentage points larger than Obama’s win against Republican Mitt Romney. It was more.

Observers expected this year that Casey would once again break his party’s record, potentially allowing Vice President Kamala Harris to survive despite narrowly losing to Trump in the Keystone State.

This formula failed to save Casey for two reasons. First of all, he didn’t outperform Harris. Casey is on track to win 48.5% of the vote; That’s the same share Harris won in Pennsylvania. He might even get slightly fewer votes than her once the counting is over, which would be a shocking result considering Casey’s track record.

” READ MORE: Donald Trump won Pa. with more votes than any statewide Republican candidate in history. Here’s how he does it.

The incumbent’s other problem was this: Trump performed better than expected in Pennsylvania. In Wisconsin and Michigan, two other “blue wall” states, Trump won by less than 1 percentage point and Democrats retained their seats in the Senate. In Pennsylvania, Trump appears to have won by about 2 percentage points.

But there was still hope for Casey because McCormick, who received 48.9% of the vote as of Friday afternoon, underperformed Trump, who received 50.5%. So even though Casey didn’t get any more votes than Harris, she still appears to have edged her by 1.5 percentage points in terms of her margin, mainly due to having a less popular opponent, and she still could have won.

Had Casey significantly outperformed Harris, she would have been re-elected.

Casey outperformed Harris in the Rust Belt but not in Philly

Casey’s margins were better than Harris’ in many post-industrial counties, such as Lackawanna County, which includes Casey’s hometown of Scranton.

But his margins trailed the vice president in the three counties that give Democratic candidates the biggest advantage in Pennsylvania: Philadelphia, Montgomery and the Allegheny, which includes Pittsburgh.

In Philly, for example, Casey has received 78 percent of the votes counted so far, while Harris has received 79 percent. This 1 percentage point difference amounts to more than 26,000 votes; But part of this difference is due to more people voting in presidential elections.

Meanwhile, Bucks County, which continues to earn its reputation as one of Pennsylvania’s most purple counties, appears to have backed Trump and Casey. Thousands of provisional, military and overseas ballots still remain to be counted in Bucks. But if Trump’s lead continues, Trump would be the first GOP presidential candidate to lead the district since 1988.

Overall, Casey is poised to win 12 of Pennsylvania’s 67 counties. But his vote share was down in all counties compared to the last reelection campaign in 2018, which was a strong year for Democrats.

His support has waned most dramatically in Greene and Fayette counties, deep in Trump country on the West Virginia border.

McCormick connects Casey to Kamala Harris

One reason Casey failed to surpass Harris was because McCormick worked tirelessly to tie Casey to the vice president.

The Republican and her allies flooded the airwaves with ads calling Casey a “weak career politician” who would “rubber stamp” Harris’ agenda, noting that Casey almost always votes with her and President Joe Biden’s administration.

Casey, meanwhile, tried to play defense with ads touting his compliance with Trump’s protectionist trade policies.

(Conversely, Casey’s aggressive attack ads against McCormick — questioning his ties to Pennsylvania and highlighting McCormick’s former hedge fund’s investments in China — may be why the Republican has failed to keep pace with Trump.)

” READ MORE: Why has Senator Bob Casey changed his stance on abortion and gay marriage, and why isn’t his opponent talking much about it?

Another reason Casey may not be able to overtake the Democratic Party may be: The incumbent’s policies are now more in line with his party’s orthodoxy More than when he began his Senate career as a “pro-life Democrat” who opposed new gun regulations and gay marriage. Although he is still to the right of his party on energy issues, he now supports marriage equality, a national guarantee of abortion rights and new gun control measures.

McCormick benefited from Wall Street-funded super PAC

Although McCormick has Trump’s coattails and a seemingly successful messaging strategy, none of it would be enough without the money. And McCormick had a ton of it, thanks to a Wall Street-backed super PAC.

” READ MORE: Meet the billionaires backing Republican Dave McCormick’s U.S. Senate bid

Keystone Renewal PAC was the only outside spending group dedicated solely to the Pennsylvania Senate race. The majority of the money spent in the race came from the candidates’ campaigns and parties.

Challengers to long-time incumbents often have trouble raising money. But McCormick is the former CEO of Bridgewater Associates, the world’s largest hedge fund, and has connections to some of the world’s richest people.

Many of those who donated to Keystone Renewal know McCormick personally, and the PAC’s spending has helped make it financially competitive in the $300 million-plus race.

The PAC spent about $54 million, according to the government transparency organization Open Secrets. About $41 million of that was spent attacking Casey, while $13 million was spent supporting McCormick.

Keystone Renewal’s biggest donor was Ken C. Griffin, the billionaire owner of Citadel Securities, a financial services company. He also received millions from Jeff Yass, Pennsylvania’s richest man and an ardent supporter of the “school choice” policies that McCormick supports.

Staff writer Katie Bernard and graphics editor John Duchneskie contributed to this article.