Growing up, Brian Donnelly followed his father and became a Republican. As an adult he voted for George W. Bush and John McCain for president.

But the Moorestown resident supported Barack Obama's reelection, and when he was left to pick between Hillary Clinton and Donald Trump in 2016, Donnelly, 42, wrote in moderate Republican Gov. John Kasich of Ohio.

Under President Trump, Donnelly's break with the GOP is complete. After first becoming an independent, he registered as a Democrat last year and is running for township council in his affluent South Jersey town, part of a suburban backlash against the president.

"It's an integrity thing. I don't believe that they represent what they have stood for in any way since I was a child, since I saw Ronald Reagan rise up," said Donnelly, a corporate attorney who is also a lawyer in the Army Reserve. He added that Republicans "have put their heads in the sand about what they are about, and I'm not going to be part of it."

People like Donnelly represent a potential Republican nightmare in this fall's fight for the U.S. House. College-educated, well-off suburban voters have been trickling away from the party for years — but Trump might make it a torrent.

"A Republican suburban purge" is what retiring Rep. Ryan Costello, a Chester County Republican, saw. "Trump accelerated substantially a trend that was already out there."

If he's right, and suburban Republican voters stay home on Election Day or cross over to the Democratic side, the results could be devastating to the GOP's chances of holding the House, because most of the key races are in moderate suburban areas, including several around Philadelphia.

Donnelly, for one, lives in one of the country's most competitive congressional districts, New Jersey's Third, which spans Burlington and Ocean Counties. Republican Rep. Tom MacArthur is being challenged there by Democrat Andy Kim.

Costello said the voter shift is not about the president's policies — which he said many right-leaning voters support — but his abrasive behavior, which polls, election results, and local GOP leaders say has repelled college-educated voters, especially women.

"Anecdotally I've heard this any number of times: 'I've been a Republican my whole life, but I'm just not doing it this year,' " Costello said.

National polls show that the vast majority of Republicans support Trump, and Burlington County GOP chairman Bill Layton argued that a strong economy could help local candidates overcome concerns about his style. But with Democratic enthusiasm high, even a small slip from the Republican base or right-leaning independents could be crippling in tight races.

Warning signs building
There have been warning signs. Special elections for Congress and state offices have seen Democrats soar past their typical performances in suburban areas. Democrats won historic victories last year in campaigns for Bucks, Chester, and Delaware County row offices, in some cases capturing positions they hadn't held since 1799. The Democrat won Virginia governor's race in 2017 with a blue wave in the Washington suburbs.

"What we're seeing is that a lot of the slide from your college-educated, white suburban voters — they do not like his caustic style and in the end that turns them away," said Mark Dion, a Republican consultant based in Washington. "They don't want America to be the hateful Twitter rants that sometimes come out of Trump World."

Pennsylvania and New Jersey are flush with just those kind of affluent, highly educated suburban areas and are therefore two of the most critical states to control of the House, along with California, said Kyle Kondik, who analyzes House races at the University of Virginia.

"If Democrats get a majority in the House, it's mainly going to come from those places," said Larry Sabato, another University of Virginia analyst. "It's those purple suburban places that leaned Republican for a long time, voted Republican pretty consistently, but a substantial portion of voters, led by women but also including some men, just don't like Trump."

The suburban response is a mirror image of Trump's success in white, working-class areas such as Luzerne County in Northeast Pennsylvania and Westmoreland County in the west, where the president won over blue-collar Democrats, becoming the first Republican to carry the state since 1988.

Most suburbs moved in the opposite direction. After Republican Mitt Romney won Chester County, Costello's home, by 529 votes in 2012,  Clinton won it by 24,606.

Democrats seek big gains in Pennsylvania, New Jersey suburbs
Democrats are now seen as heavy favorites
to pick up open seats based in Chester, Delaware, and Atlantic Counties, and are taking aim at three more districts in the region. Two suburban seats in North Jersey are also seen as prime opportunities for Democrats.

In the most optimistic scenario, Democrats' combined gains in New Jersey and Pennsylvania could approach half of the 23 seats they need to net to have a House majority.

MacArthur and Rep. Brian Fitzpatrick, of Bucks County, are the only GOP incumbents running in the Philadelphia area, and both face tight races. Each has tried to tout their independence from their party and support from, or cooperation with, Democrats.

There were similar concerns about Trump and the suburbs in 2016, yet Republicans kept every one of their seats in the Philadelphia region, signaling that traditional Republican nominees — including MacArthur and Fitzpatrick — could still succeed.

Four Republicans in the region, however, have since opted to retire, depriving their party of candidates who established identities apart from the president.

Polls suggest that education levels, which are higher in the suburbs, are a clear dividing line on Trump.

In five competitive suburban races in four states polled by Monmouth University, white independent men without a college degree preferred the Republican candidate 2-1. White independent women with a degree preferred the Democrat by the same margin. (Among nonwhites, educational attainment did not correlate as closely with preference.) Those numbers come from districts that include Fitzpatrick's and MacArthur's.

The surveys offer one explanation for why Chester County swung in 2016. Slightly more than 50 percent of adults there have a college degree, 20 percentage points higher than the national average.

`Congress is supposed to be a check'
Though Trump remains strong with the GOP base, swing voters are also at risk.

Consider Susan Coleman, a Moorestown Democrat who voted for Clinton and her Republican congressman, MacArthur, believing in a balance of power.

After Trump's election, she attended the Women's March in Washington and became politically active for the first time. In her 60s, she is leading a grassroots liberal group pushing to unseat the congressman she voted for, arguing that he has been too aligned with Trump, playing key roles in the president's tax plan and push to roll back the Affordable Care Act.

"Congress is supposed to be a check … and it has abdicated its responsibility," Coleman said. "So many people who have always, always voted Republican now come up to me and express their chagrin about what Donald Trump is doing."

Mark Harris, a GOP consultant based in Pittsburgh, said Republicans can hold on to right-leaning suburban voters if they can make Democrats an "unacceptable" alternative.

"They didn't wake up and go from being mainstream Republicans to left-wing Democrats," Harris said.

So Republicans have taken sharp aim at Democratic contenders, accusing Bucks County's Scott Wallace of wanting to raise taxes and questioning the national security credentials of Kim, MacArthur's challenger.

And in some parts of competitive districts, Trump could be a positive for GOP candidates. With a big margin in the party's Ocean County stronghold, Trump won MacArthur's district by six percentage points. It was the president's best showing in any suburban seat outside Philadelphia.