This analysis was first available to Bloomberg Government subscribers.
Lamb, a 33-year-old Marine veteran and former prosecutor, has positioned himself as more representative of the district than Saccone, a 60-year-old state lawmaker. A Democrat has not represented the district since 2002 when its boundaries were redrawn.
Democrats need to flip 24 GOP-held seats to claim a House majority.
If Saccone wins, his re-election in the fall will not necessarily be any easier, since a recent Pennsylvania Supreme Court decision means the district in which he runs in November will be less Republican than the district as it is now drawn. Former Vice President Joe Biden called the tax cuts "obscene" during a campaign event with Lamb last week.
"We're very concerned about this race", one national Republican involved in the campaign told TPM.
"This place had 400 employees and they added 80 new full-time employees since tax reform passed", Trump Jr. said as he ate a bowl of Sarris's apple pie ice cream.
The newspaper says Democrat Conor Lamb is impressive, but argues that an upset in Pennsylvania could start a "Democratic wave" that would lead to President Donald Trump's impeachment.
The group confirmed the existence of the mailer to ABC News but declined to describe it or where it was sent in the 18th Congressional District. Lamb touts his resume and declares that he wouldn't vote for Democratic House leader Nancy Pelosi as party leader.
"They're opening a lot of the steel mills up because of what I did", he said.
It shows once again that, all-in or not, Donald Trump's endorsement doesn't mean as much as many Republicans would like it to mean.
Tuesday's big special election in Pennsylvania's 18th congressional district is shaping up to be a must-win for Republicans - but a new poll suggests that they are about to suffer a massive loss.
And, in the aftermath of the Parkland shooting, Lamb called for tougher background checks but not stricter gun laws. He will help me very much.
Republican groups from outside the state have given Mr Saccone donations of $9m.
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The president lent his weight to Republican Rick Saccone in the final days of a surprisingly competitive special election outside Pittsburgh that could reverberate nationally ahead of the November midterm elections.
Lamb has crystallized the debate over whether a younger, charismatic Democrat appealing to win back traditionally Democratic voters can overcome Republican party loyalty in a GOP-leaning district at a time when Trump remains a divisive figure.
On the other side of the aisle, Lamb has raised almost $3.5 million since January for his campaign directly, and outside groups backing him have spent only about $1.7 million in the race so far.
"Bring your friends and family, drag them out", he told hair-netted employees boxing up chocolate for Easter.
But while both sides have decided that the race is worth digging deep into their pocketbooks, the fact remains that, unlike Virginia and part of Georgia where candidates also went head to head a year ago, this area in Pennsylvania voted solidly and overwhelmingly Republican in 2016.
But Lamb is apparently having enough success to worry Saccone's most high-profile backer.
A key difference between Murphy and Saccone: Murphy tended to have labor union support. It's about 93% white and only about 36.5% of the population graduated from college. While Democrats outnumber Republicans on the voter registration rolls, this part of Pennsylvania has been trending Republican in federal elections for decades.
TRUMP: That's much easier than doing what I have to do because - but this is much more effective.
National Republicans have done everything possible to save the district.
But the rest of the district is composed of more rural areas that strongly support gun rights, including parts of Washington, Greene, and Westmoreland Counties.
Pennsylvania's 18th also is a district that will soon cease to exist in current form. Registered Democrats still outnumber Republicans by nearly a four-to-three ratio in a district where gun rights are a high priority, and Democrats still hold some local offices.
Lamb has largely, but not entirely, avoided tying himself to national Democrats.
Manley pointed to the string of special elections and governor's races that have gone the way of Democrats in recent months.