It’s being called the Republican “Trumpmare” and it looks like this: Not only will a Donald Trump nomination cost the GOP the White House, but the Senate as well – and possibly even the House, though that’s a long shot.

The fear is that in a general election, many Republicans won’t show up for the Donald, and/or that Democrats will come out in force against him.

If that happens, the thinking goes, say goodbye to the five incumbent GOP senators in states such as Wisconsin and Pennsylvania that went for President Obama in 2012. Probably also forget the Republicans vying for open seats in Nevada and Florida that also went for Mr. Obama last time.

Democrats need only four GOP seats to retake the Senate – five if they lose the White House. They would need 30 seats to win back the House. With gerrymandered safe districts, that’s a very steep hill to climb, but Mr. Trump at the top of the ticket could endanger GOP seats in suburban swing areas such as this one, north of Philadelphia.

Heather, a moderate Republican voter in Pennsylvania, personifies the possible peril. One day her son was exclaiming in another room, “I am Donald Trump! I am Donald Trump! I am so mean!” She went into the room and found him holding his Darth Vader action figure, bouncing it around.

“I don’t know what to do with that,” she tells a reporter, asking that her real name not be used because she is known in Republican circles.

Heather says that she and her moderate Republican friends are distressed about how to vote if Trump wins the nomination. “We’re all talking about it,” she says. She insists, though, that they won’t vote for a Democrat – certainly not Hillary Clinton, she says, citing the trust factor.

'We've never seen anything like this'

Trump says there’s nothing to worry about – that he’s actually widening the tent, converting Democrats to his brand of Republicanism and bringing in new voters. In Pennsylvania, more than 60,000 Democrats have re-registered as Republicans.

“The truth is, no one knows” whether a Trump nomination will have a negative down-ballot effect on other races, says Jennifer Duffy, who tracks Senate races at the independent Cook Political Report.

That’s because this race is unlike any other, she says. “What we lack here is data, because we’ve never seen anything like this.”

The closest analogy is 1980, when a landslide victory by Republican Ronald Reagan crushed Senate Democrats and flipped control to the GOP. The reverse could happen this time. In a match-up with Mrs. Clinton, the Democratic front-runner, Trump loses badly – by 11 points, according to the Real Clear Politics average of polls. He loses by a wider margin – 17 points – to her challenger, Vermont Sen. Bernie Sanders.

But Ms. Duffy says that 1980 is not really comparable. At that time, Senate Democrats didn’t see the threatening wave coming. Now, Republicans do, and they and their outside donors are busy erecting a cash and advertising “firewall” to protect their congressional candidates from the flames of Trump, according to The Washington Post.

Also, unlike Reagan, Trump is not ideologically consistent, making voter behavior harder to predict.

One “thread of evidence” that should encourage Republicans, says Duffy, is that Senate Republican incumbents have done well in primaries so far. There’s not an anti-incumbent mood in the Senate, even with Trump on the ballot. But the real test will be in a general election, when Democrats and Republicans go head-to-head.

Another encouraging sign for Republicans is new voters or crossover voters attracted to the billionaire. “Trump could actually be helpful” in blue-collar states such as Pennsylvania and Ohio, where white, working-class males like his message, Duffy adds.

Democrats for Trump

Take Bill Dostal, a retired truck driver who is just emerging from the WaWa market here in Doylestown. He’s a registered Democrat who voted for President Obama in 2012. But he’s seriously considering Trump this time.

Mr. Dostal “can’t stand Hillary,” and though he finds Republican John Kasich “more level-headed” than Trump, he likes the billionaire’s plain talk.

“Trump is saying what most people feel,” says Dostal, carrying a gallon of milk in his hand as he stops to talk before getting into his pick-up truck. “We’re just tired of everything.”

Simon Williams, also retired and a registered Democrat, feels similarly. If Ohio Governor Kasich were ahead, he’d get his vote, but Trump “is telling it like it is.” Mr. Williams likes the real estate mogul’s business record, his toughness.

Standing outside the Quaker Steak & Lube restaurant in Mechanicsburg, Williams says he’s fed up with both parties. “Forget about Democrats and Republicans. Get someone in the White House who’s going to work for the people.”

But analysts point out that Trump is making too much of his ability to attract new voters and crossover voters to the party.

Record GOP turnout in the primaries also has to do with the number of competitors and the stiffness of the competition, reports Monitor political writer Linda Feldmann. Strong Republican turnout now doesn’t necessarily translate to the general election.

In Pennsylvania, Preston Maddock describes the GOP talking point that 63,000 Democrats have switched their registration to Republican as “a red herring.”

Mr. Maddock, the communications director for the state’s Democratic party, points out that 29,000 voters have done the opposite, that Democrats have registered more new voters than Republicans have, and that more voters have moved from independent to Democrat than from independent to Republican.

The net effect is a GOP boost of only 15,000 voters. In a state where Democrats enjoy a 1 million voter registration advantage, “that’s a meaningless difference,” he says.

Meanwhile, a recent Franklin & Marshall College Poll has Mrs. Clinton beating Trump in a hypothetical match-up by 46 to 33 in Pennsylvania.

This is not lost on Don Petrille, who last week gathered with a few of his fellow Republicans at the Bucks County GOP headquarters here to meet briefly with Sen. Pat Toomey. The Republican senator is running for reelection this year, and is considered vulnerable in a state that hasn’t voted for a Republican president since 1988. The senator admits he's in "a tough race."

Mr. Petrille is very concerned about the “Trump effect” on down-ballot races, not to mention on the presidential race. Republicans in swing Bucks County have worked hard to get Republicans elected at the state, county, and judgeship levels.

Then there’s the open seat in Petrille’s district created by the retirement of Republican Rep. Mike Fitzpatrick – rated a “toss-up” by the Cook Political Report.

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“All of these gains could be for nothing if there’s a backlash against the Republican Party because of Donald Trump,” says Petrille. “I think Trump could be a 40-state disaster.”

Petrille is backing Republican Ted Cruz instead. The conservative Texas senator doesn’t fit the county’s moderate stance, but Petrille is hopeful. “I think the more people see of Cruz, the more they’ll like him.”