Beneath the surface of Pennsylvania’s mostly sleepy, boring, low-turnout primary on Tuesday was a trend that has stalked Joe Biden and Donald Trump across the country over the past month: a substantial protest vote.

This time though, the candidate who drew more attention for the protest they faced was not the current president but the former one. 

Trump faced about 36,000 more protest votes than Biden, even though Biden faced an organized protest campaign from left-wing critics of his policy on Israel and its military campaign in Gaza. What’s more, Pennsylvania is the only frontline battleground state to hold closed primaries since Biden and Trump clinched their party’s nomination, meaning only registered party voters were able to vote for — or against — their party’s presumptive nominees. 

That was particularly noteworthy on the Republican side, after Trump allies repeatedly attributed earlier swing-state protest votes to independents and Democrats who were able to cross over and vote in open primaries, as opposed to any intra-party blowback.

More than 163,000 Republican voters cast ballots for former U.S. ambassador to the United Nations Nikki Haley, who dropped out of the presidential race last month, or submitted a write-in ballot. On the Democratic side, at least 127,000 voters either voted for Rep. Dean Phillips, D-Minn., who also ended his presidential bid last month, or cast a write-in vote.

To be clear, the electorate that showed up Tuesday — when few contested races were even on the ballot — is much different than the coalitions that show up in November. But in a state Biden won by roughly 80,000 votes in 2020 and Trump won by about 45,000 in 2016, there could not be more riding on each candidate’s ability to bring their internal dissenters back on board — or at least keep them from joining their rival.

J.J. Abbott, a Pennsylvania-based Democratic strategist who worked for former Gov. Tom Wolf, said the Tuesday results offer insight “into the damage that Trump still needs to fix within his own party.”

“I certainly believe that President Biden has some work to do with some of his base as well,” Abbott said. “But I think this really exacerbates the challenge for Trump. Trump has a lot of damage control to do that doesn’t really seem to get the attention that some of the challenges that Biden is trying to navigate do.”

The Trump campaign believes news of the protest vote has been greatly overblown by its opponents. For starters, recent surveys of Pennsylvania have shown a close, tightly contested race there, including April surveys by Fox News and Bloomberg/Morning Consult, which also showed Trump leading in nearly every other contested battleground. State and national surveys, including NBC News’ recent poll, also show that about 9-in-10 self-identified Republicans back Trump in the general election, more than the 83% who voted for him in the GOP primary in Pennsylvania on Tuesday.

There was also internal frustration on the Trump campaign over the way Pennsylvania counted write-in votes, tabulating them last — which gave the initial appearance that the anti-Trump protest vote was significantly larger than the anti-Biden vote than it actually was.

And, as a senior Trump official said, the Trump team had yet to spend any money in Pennsylvania for the primary — whether it be on the ground or over the air — while Biden had opened up campaign offices and put ads up on TV.

“We haven’t run anything. We haven’t done anything,” this person said. “We haven’t spent a nickel in Pennsylvania, nothing. The biggest difference they get is [36,000] more protest votes, if you want to call them that.”

This person said it will still take time to figure out who comprises the Haley-til-we-die coalition that’s still voting in Republican primaries for a long-departed candidate — how many are 2020 Biden voters, how many will return home to Trump in November, and how many still need to be persuaded.

“It’s not going to be hard for them to vote for Donald Trump in November when they’re looking at economic and inflationary numbers that are worse than what they are today,” the senior Trump official said. “And the campaign in Pennsylvania hasn’t even been prosecuted yet.”

Karoline Leavitt, a Trump campaign spokesperson, said Trump “delivered a resounding primary win in Pennsylvania,” adding the “dishonest Biden campaign has spent millions in Pennsylvania gaslighting voters.”

But Democrats say the Trump campaign’s limited efforts so far in Pennsylvania are part of why Biden is now in a better position there.

Plus, they say they’re the only side that had to deal with an active, organized effort that was actually pushing voters to write in an “uncommitted” vote, whereas there was not organizing on the GOP side to get voters to cast ballots for Haley.

“The difference is that the Biden campaign, at least in Pennsylvania, is doing the work that’s going to be necessary to” unify the party, “which is building local infrastructure, opening offices, getting people out, having conversations,” Abbott said, adding, “The last time that Republicans in Pennsylvania won a major statewide election was in 2016. Trump has to rebuild that coalition, which no one has been able to do — including Donald Trump in 2020 — since 2016.”

‘A warning sign’

Where these votes were cast also mattered, both Democrats and Republicans said. In the collar counties around Philadelphia — key suburban territory presidential campaigns have fought for for a generation — Haley was garnering 20% to 25% of the vote, well above her statewide total of 16.5%. She was also floating around 20% in Erie County, the quintessential swing county in the state, and in Lancaster County, a pro-Trump area that the Biden campaign is targeting to chip away at this fall.

“I think it’s a warning sign,” one Pennsylvania-based Republican strategist said, noting that Sen. Bernie Sanders, I-Vt., picked up a large chunk of the primary vote there against Biden in 2020 even after he had left the race.

“I don’t think it’s the end. But if you’re the Trump campaign, you have work to do in the suburbs,” the strategist continued, while adding: “Most of them will end up voting for Trump.”

NBC News spoke with two Haley primary voters in Pennsylvania on primary day — and their answers bolstered that point. One, Jim Nixon, said he would ultimately vote for Trump in the fall and condemned the criminal hush money trial the former president currently faces in New York, calling it a “kangaroo court.” The other, Joan O’Donnell, said she voted for Biden in 2020 but would probably vote for Trump this time around — unless he was convicted.

For Democrats, the largest chunk of write-in votes came in Allegheny and Philadelphia counties — home to Pittsburgh and Philadelphia. Depressed turnout among Democratic constituencies there could have disastrous effects on Biden’s ability to win.

“I do think the Democratic Party is at least marginally more unified,” the Republican strategist said. “But again, all of these things are fixable.”

Former Rep. Keith Rothfus, R-Pa., said given how small the margin of victory was in both 2020 and 2016, the protest votes take on an extra level of importance. Typically, Rothfus said, Trump getting 83% of the vote would be considered a landslide win.

“In a lot of these rural counties, 10% or more were voting against Trump,” he said. “That should be, like 98% territory.”

Ultimately, Rothfus said it’s Biden who has more to worry about with the protest vote, pointing to the continued Democratic divide over how the war between Israel and Hamas has played out.

But another factor that has Democrats feeling good about the anti-Trump vote in the GOP primary is that recent electoral history has shown an important slice of Pennsylvania Republicans are open to voting for Democrats. In 2022, Republican turnout in Pennsylvania was solid. The party’s problem was that too many Republicans voted for Democratic Gov. Josh Shapiro and Sen. John Fetterman, D-Pa.

Exit polling at the time showed Shapiro won 16% of Pennsylvania Republicans, while state Sen. Doug Mastriano, the GOP gubernatorial nominee, won just 3% of Democrats. (In 2020, the split of cross-party voters was almost evenly divided between Biden and Trump.)

“There’s a real group of Pennsylvanians, in this case, registered Republicans .. who are continuing to reject Trump and his brand of politics, his brand of chaos and extremism,” said one Democrat who worked on a 2022 effort in the state. “And that is a really important piece of the puzzle for November.”

The Biden campaign authored a memo boosting the results following the Tuesday vote, writing that the former president’s “general election problems get worse,” with campaign spokesman Ammar Moussa calling the Pennsylvania primary additional evidence that he “has no path to building the coalition necessary to win 270 electoral votes.”

The Trump campaign appeared to fire back in a Thursday memo of its own, blasting Biden as having “many problems” — including with the Democratic base.

“There is lots of dissension in the White House,” top Trump campaign advisers Chris LaCivita and Susie Wiles wrote. “There is dissension on the issue of Biden’s handling of the situation in Israel — where the majority of Americans support Israel’s right to self-defense.  There are radical staffers who are pushing pro-Hamas positions and regularly engaging with those pushing anti-Semitic messaging in the United States and elsewhere around the world.”

Others still felt the whole ordeal was much ado about nothing. Lou Capozzi, the GOP chairman in Cumberland County, Pennsylvania, where Haley won more than 21% of the Republican primary vote, said he thought focus on the anti-Trump and anti-Biden protest votes was overblown.

“I honestly don’t make much of it,” he said. “At the end of the day, those people will come back to roost.”