The men are competing to complete the 18th District term of Tim Murphy, who resigned in a sex scandal. In 24 consecutive special House elections, the party defending the vacant district has been victorious, according to data compiled by Bloomberg Government. The president himself appeared with Saccone in early February, and then returned to the district Saturday night for one of his patented "campaign rallies".
"We're very concerned about this race", one national Republican involved in the campaign told TPM.
Differing from the position of the AFL-CIO, the largest federation of unions in the U.S. which supports Medicare-for-All, Lamb says that single-payer health care system hasn't proven to be effective and prefers to focus on fixing the Affordable Care Act.
Trump said Republicans should not "rest on their  victories" and get out and vote for people like Saccone and in the 2018 midterms.
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.
Particularly worrying for Republicans is that Lamb has managed to parry the attacks they've hoped will work across the map.
Trump also said that the country should discuss having drug dealers face the death penalty, as they do in China and elsewhere because they are responsible for thousands of deaths.
And, in the aftermath of the Parkland shooting, Lamb called for tougher background checks but not stricter gun laws. Saccone emphasizes that he's a staunch opponent of abortion rights. "I just don't understand that people say we shouldn't put lobsters into hot, boiling water. but we can kill babies". He outraised Saccone by a almost five-to-one margin, hauling in $3 million by the most recent campaign finance reports filed February 21. The district's voters are far more likely to see television ads from Washington-based Republican groups than from Saccone.
However, when it comes to campaigning, Saccone diverges with Trump's successful strategy of running as an outsider, instead boasting of his decades of experience in Pennsylvania politics. It was part of the fund's $3.5 million investment in the race, about a third of the total that outside GOP forces have injected.
"The tax cuts are helping them expand, giving them a little boost", said Saccone.
Lamb has highlighted protections for Social Security and Medicare in a district with a median age of 44.6, compared to 37.7 nationwide, and where 36.3 percent of households receive Social Security, compared with 30.2 percent in the USA, according to estimates from the Census Bureau's American Community Survey. It's about 93% white and only about 36.5% of the population graduated from college.
"God knows, if it's going to make it hard for Trump, the media's going to be all over it", said Trump.
The district has education and income levels higher than the national and Pennsylvania averages in part because it includes some affluent parts of Allegheny County near Pittsburgh.
Vice President Mike Pence says Tuesday's special election in a western Pennsylvania House district is "about a whole lot more than the two candidates" on the ballot.
Pennsylvania's 18th also is a district that will soon cease to exist in current form.
Win or lose tomorrow, Lamb could run in November in a reconfigured district, numbered the 17th, where Rep. Keith Rothfus (R) is expected to seek re-election. The Pennsylvania Supreme Court has struck down the map, and Lamb is likely to run next fall against Rep. Keith Rothfus (R-PA) in a much less conservative district than the gerrymandered one he's now trying to win.
Trump stumped alongside Saccone on Saturday, blasting "Lamb the sham" and pushing for support of Saccone because "the world is watching". He could face opposition in the May Republican primary.
Polls open at 7 a.m. Tuesday and close at 8 p.m.