A new poll of one of Pennsylvania's most critical congressional races shows Democrat Conor Lamb with a large lead over Republican Keith Rothfus, and reflects several trends that could weigh heavily in similar contests outside Philadelphia.

In a Republican-leaning swing district just outside Pittsburgh, Lamb has a 51 percent to 39 percent lead among potential voters — those who have participated in an election since 2010 or are newly registered to vote, according to the survey from the Monmouth University Polling Institute. His lead is even larger in a model that predicts a Democratic turnout surge.

The race is unusual in several respects. Lamb has an unusually high name ID due to his nationally watched special election win in March; the contest features two incumbent congressmen drawn together by Pennsylvania's new congressional map; and those new districts have undercut Rothfus' advantage in experience by throwing in a wave of new voters who don't know him — and lean far more left than those in his old, safely Republican district.

But other trends showing a Democratic enthusiasm advantage, driven by suburban disapproval of President Trump, are consistent across the swing districts that Monmouth has surveyed, said Patrick Murray, director of the polling institute.

"We're seeing that in every race we've polled," Murray said, listing off seven competitive races, including ones in Bucks County, Ohio, Virginia, and West Virginia. The race in Pennsylvania's new 17th District also could offer parallels to other moderate swing districts around Philadelphia.

In the Lamb-Rothfus race, the survey found Republicans are also hurt by some of Trump's signature economic policies, such as his tax cuts and tariffs, a finding which could be devastating if spread across other races. On the same morning the poll was publicly released, the top House analyst at the Cook Political Report, an election forecasting site, changed its rating on the race from a tossup to "lean Democratic."

Much can change, of course, by Election Day, but the poll could be a window into the atmosphere surrounding key races in November's fight for control of the House.

Among them:

— Disapproval of Trump is driving Democrats, and outpaces the president's support in these kind of relatively balanced districts. Overall, 44 percent approved of the president's job performance and 51 percent disapproved. But the depth of feeling weighs on the Democratic side: 43 percent strongly disapproved against 28 percent who strongly approved of Trump.

"Without this disapproval of Trump, you wouldn't see these moderate, right-leaning voters possibly staying home and you wouldn't see this swell of Democratic voters coming out," Murray said. "Democratic enthusiasm is similar no matter where you are. The question is: Are there enough disenchanted Republican-leaning voters who are willing to vote for a Democrat, or stay home?"

— Trump is a driving factor: 63 percent of those surveyed said it's very important for them to cast a vote that shows how they feel about Trump. Again, the feeling was stronger among his critics: 73 percent of them felt that way, against 59 percent of Trump supporters.

Democrats' House leader, Nancy Pelosi, could be a drag on Lamb as well — 21 percent of voters said he is too close to her — but she is not as big a factor as the president, and it's easier for the Democrat to create distance from the Californian (as Lamb vocally did in his special election campaign), Murray said.

— Key Trump policies aren't playing well in this race: Nearly half of voters (48 percent) in the race say the president's trade and tariff policies will hurt the economy in that area, while 31 percent say the policies will help. On the GOP tax plan, there is a slight Republican advantage: 43 percent approve against 39 percent who disapprove. But 31 percent think their tax bill will likely go up, against 22 percent who think it will go down (even though nonpartisan estimates predicted that the vast majority of taxpayers would pay less, at least initially).

Republicans have long hoped that their tax bill would help stir enthusiasm among their voters, but if that fails, they may be short of selling points. Lamb vocally campaigned against the tax bill when he won his special election race in a district that voted overwhelmingly for Trump.

"It comes down to bread-and-butter issues," Murray said. "This is one of the reasons why Lamb won the [special election]. Folks there were concerned about trade policies, tariffs — they don't play well in this district. We're finding that in other industrial districts that we're polling."

There are some key factors, though, that make this race different from other swing districts, and which can't be easily replicated elsewhere:

— Lamb's high profile: He's only been in Congress for a few months, but the Democrat is more well known and more popular than his three-term rival: 44 percent have a favorable impression of the 34-year-old newcomer, while 17 percent see him unfavorably. For Rothfus, those numbers are 31-23. Lamb's notoriety is likely a residual effect of his special election, in which millions were spent on the air, reporters from around the country poured into Western Pennsylvania to cover the campaign, and Trump made a personal visit to campaign for the Republican nominee, State Rep. Rick Saccone.

Rothfus' campaign pointed to that attention in predicting a shift as the race heats up.

Lamb "was the beneficiary of millions of dollars of advertising just a few months ago which falsely painted him as a moderate Democrat," emailed Rothfus spokesperson Mike Barley. "The more voters know, the more likely they are to choose Keith Rothfus."

— New district: Rothfus also lost much of the advantage of incumbency due to the new congressional map, drawn by the Pennsylvania Supreme Court to replace what it ruled were unconstitutionally gerrymandered districts.

Only 56 percent of the electorate now comes from Rothfus' old, strongly Republican district. One-fifth of the electorate came from a strongly Democratic district based in Pittsburgh and another fifth is from Lamb's former district, according to Murray. Roughly a quarter of the voters there have never lived in a district represented by either Rothfus or Lamb, which again points to the weight of the special election last March, which dominated the news in the region.

The poll surveyed 401 voters in Pennsylvania's 17th  District from July 19 to 22. The margin of error is 4.9 percentage points for the full sample and 5.2 percentage points for the likely voter models. The error of the gap between the two candidates' vote share is 6.8 percentage points.