He said it was crucial for Republicans to muster forces to turn back a strong Democratic drive to win one or both houses of the US Congress in November midterm elections.
While the western Pennsylvania district normally leans Republican, Democrats are confident they can win Tuesday's election, in part because of the controversy surrounding Trump's term in office. Should Lamb win, it would be another indication of a hard road ahead for the GOP in this year's midterms.
Mr Trump also urged a crackdown on sanctuary cities and vowed to toughen enforcement at USA borders and to root out members of MS-13, a criminal gang. "I don't know much about Rick Saccone", he acknowledged, adding that he remains skeptical about Trump.
Trump referred to Lamb as "Lamb the sham".
"There's also a segment of undecideds that hopefully they'll remember why they voted for Trump to begin with, and they'll come out and vote for me", Saccone said before talking to natural gas workers in Canonsburg.
"What Trump is doing is good for the country", she said, "and Rick's going to support everything he's doing". "Here's the thing: We're dealing with people that want to obstruct". He said Saccone "doesn't have a natural fundraising base [and] has never really had to do it" in his campaigns for the state legislature.
The local AFL-CIO is calling members and going door-to-door to ensure they vote for Lamb on Tuesday.
"If the Democrats are able to come close, or win this, it will send shock waves across the political world", Kevan Yenerall, a political science professor at Clarion University, said in an interview in the town of Mount Lebanon.
He then launched into a robotic imitation of someone being "presidential".
"Do me a favor".
Trump was in the state to stump for GOP special election candidate Rick Saccone, but only mentioned him in passing.
"We catch a drug dealer, death penalty".
The president noted that both Lamb and Saccone both have endorsed the tariffs.More news: Taylor Swift blows the lid off in latest video 'Delicate'
"This should've been handled, by the way, over the last 30 years".
Trump also said he hoped to run for re-election against Democrat Oprah Winfrey, although the entertainer has ruled out a run. "It was great to see North Korea going and participating".
Trump talked up the economy since his election.
The president criticized Democrats for blocking his long-promised border wall and attacked so-called sanctuary cities that refuse to cooperate with federal immigration authorities. This proposal echoed recent remarks he made during a White House opioid summit.
Critics have said the tariffs could trigger retaliatory trade measures and damage the USA economy. "We're gonna tax Mercedes Benz. We're gonna tax BMW".
Trump talked about trade with Mexico and Canada and the ongoing NAFTA negotiations.
Trump's appearance was part of a White House push to help Saccone avoid a once-unlikely defeat.
According to the Guardian, he began the segment by praising the criminal justice system in China - and even said he got the idea for the "discussion" from Chinese President Xi Jinping. Trump called the network "fake as hell CNN".
But in classic Trump fashion, he quickly steered away from his main reason for being there. The president tweeted Friday night that he is looking forward to speaking publicly in support of the GOP candidate.
Trump also mocked global criticism of the steel and aluminum tariffs he ordered this past week.
Lamb's response to the 75-minute event was muted.
"Words matter", he said on NBC's "Meet the Press", whose host, Chuck Todd, had been the subject of some of Trump's most virulent tongue-lashing, including being called a "sleeping son of a bitch". Even some Republicans admit that Lamb, who is also a veteran, is a better candidate. "We can't say make America great again as I already did that".