So Trump and his surviving rivals are planning to campaign here, and there are signs that voter interest is high. So far this year, state officials say, 54,817 Pennsylvania Democrats have switched their registration to Republican, making them eligible to vote in the closed primary. (Slightly more than 27,000 voters moved the other way.)
That fits the race's pattern thus far: Overall, the GOP's primaries and caucuses across the nation have drawn higher turnout than the Democrats'. In part, polling indicates, it's the natural pendulum swing of U.S. politics, as voters grow frustrated with the party holding the White House and yearn for a new direction.
And this year, there's Trump, who has stormed the GOP with a populist attack on leaders of both parties, a rant against illegal immigration and job-killing "bad trade deals," and a vow to make the U.S. strong again.
How's he playing here? A meeting of Trump supporters at a restaurant in Newtown, Bucks County, drew 240 people Wednesday night, organizers said. When Inquirer reporters surveyed Montgomery County voters who'd switched from "D" to "R," some cited their disappointment with President Obama; others, their interest in Trump.
Several, such as retiree William C. Bauer of Harleysville, mentioned both.
Bauer, retired from the construction industry, said he backed Obama in 2008. Now he blames the Affordable Care Act for the rise in his ailing wife's medical costs. He counts on Trump to repeal Obamacare; Cruz is his second choice.
"Trump's kind of a big mouth," Bauer said, "but I think at heart the guy means well."
Raymond S. Bibbo Jr., 57, of Eagleville, said he, his fiancee, and several friends all switched because they were fed up with the Democrats. Bibbo, a caterer, said, "Four more years with Hillary, and ISIS will be walking down Main Street."
Trump has sharply divided the GOP, though, drawing opposition from committed conservatives who dislike his dalliances with liberalism, and from mainstream party leaders who worry that his often-xenophobic and misogynist rhetoric will doom the party to irrelevancy in an increasingly diverse nation.
But this year, those mainstream GOP leaders are being schooled by the voters.
"I'm scared to say it," said Bonnie Ann Alder, a registered nurse, "but I'm a Trump supporter."
Alder, 51, is from Narberth, in the heart of the vote-rich Philadelphia suburbs both parties covet. A longtime Democrat, she switched to Republican last month out of deepening frustration with the cost of health insurance, among other things. She thinks Trump will stand strong against terrorism - and for middle-class people like her.
"I'm a hardworking person with a masters' degree, and my health insurance for my company is absolutely terrible," Alder said, ". . . and when I hear Obamacare is funding immigrants who don't have jobs, let alone green cards, it's very frustrating."
Alder favors Trump over Cruz, the Texas senator elected with a base in the tea party movement and among evangelical Christians, who's the champion of resistance to Trump from the right; and Kasich, the Ohio governor who is the sole survivor of a crop of mainstream center-right candidates, and who pitches himself as the most pragmatic and electable Republican standing.
Indeed, the split is so deep that many Republican strategists say it's increasingly plausible that the national convention will open in Cleveland on July 18 with no candidate having secured the 1,237 delegates needed to win on the first ballot. That hasn't happened since 1976 - the same year the party's Pennsylvania primary last mattered.
That would leave things up to the delegates, who in recent years have been little more than props in conventions that have become elaborate infomercials.
Pa. GOP's quirky rules
And here's a Pennsylvania twist: Getting the most votes in the GOP primary doesn't guarantee that candidate the bulk of the 71 delegates.
The state party's rules are complicated: Only 17 delegates - three party leaders, plus 14 at-large delegates chosen by the party - are awarded to the popular-vote winner, and are bound to vote for that person on the first ballot in Cleveland.
Three delegates are elected by voters in each of 18 congressional districts. They run as individuals, with nothing on the ballot to show which presidential candidate they support - if any.
"It's a two-front campaign," said Lowman Henry, a longtime conservative activist who runs a Harrisburg policy think tank and is chairman of Cruz's state effort. "We have to educate our voters to vote for Ted Cruz at the top of the ballot, because we want those at-large delegates - and also tell them to vote for delegates in their districts who are for us."
Henry said the Cruz campaign had recruited delegate candidates in all but the two Philadelphia congressional districts.
All the campaigns agree that Marco Rubio had the best organization here, with full slates of delegates in each district. Leading the Florida senator's efforts were GOP national committeeman Bob Asher of Montgomery County, and Chris Bravacos, the Harrisburg communications consultant.
But that was before last Tuesday, when Trump crushed Rubio in his home state, causing him to quit the race - while Kasich kept his hopes alive by winning in Ohio.
Kasich, raised near Pittsburgh, "is a natural candidate for the Philadelphia suburban vote," said his state chairman, former U.S. Rep. Bob Walker. He noted that Kasich beats Hillary Clinton in most polls of hypothetical general-election matchups - while both Trump and Cruz lose to the former secretary of state.
"It doesn't do us any good to nominate a candidate who's going to get slaughtered," Walker said. He also voiced a worry other Republican operatives share: that a weak candidate at the top of the ticket could doom the party's down-ballot candidates this year - starting with Sen. Pat Toomey, whom Democrats are targeting in efforts to regain control of the Senate.
But if, in the end, the GOP convention is brokered? Strategist Charlie Gerow of Harrisburg, whose experience dates to Ronald Reagan's runs, says the Pennsylvania party's quirky rules could actually be a boon in a multiballot floor fight: the 54 district-level delegates will be the largest uncommitted bloc in the hall.
Gerow said, "Pennsylvania would be ground zero, part of every scenario, every conversation, every phone call."
Gauging 'Trump effect'
To be sure, many more votes remain to be cast before the extent of what Republican analyst Nate Benefield of the Harrisburg-based Commonwealth Foundation calls "the Trump effect" is known.
In a blog post last week, Benefield pondered the high number of Pennsylvania Democrats switching parties this year.
Those numbers "may be the 'Trump effect.' . . . But it may also be the 'Wolf effect,' or the 'Obama effect,' or simply presidential election enthusiasm," Benefield wrote. "It could also be folks switching to oppose Trump."
Folks such as Kevin R. Booth, 58, of Fort Washington. He switched to Republican, reasoning that the Democrats' contest is all but over, and that he'll vote for Kasich - to stop Trump.
"I would be concerned to think that Trump could be the president," Booth said. "I don't think he has the right personality."
Such switches show voters are getting interested, Benefield wrote: "Most likely . . . it's the fact this will likely be the first time in decades Pennsylvania's primary will help determine the GOP nominee."