Speaking to a crowd of supporters in a packed airplane hangar outside of Pittsburgh, Trump gave one of his trademark speeches including a rare imitation where he feigned what it would be like to act "presidential".
Trump was in the state to stump for GOP special election candidate Rick Saccone, but only mentioned him in passing.
At one point, Trump unveiled what he said would be his 2020 campaign slogan: "Keep America Great, exclamation point".
Trump conceded Lamb might not back Pelosi but said most House Democrats will. "We need Republicans put in office", Trump later added.
Prominent Republican political figures, led by President Donald Trump, came together on Saturday to support Rick Saccone, the GOP's candidate in Pennsylvania's 18th Congressional District special election that has drawn national attention.
Trump said if he runs again - which is "almost positive" - he can not use his "Make America Great Again" tag line. He can say, 'I love President Trump.'. "I might like him", the president quipped.
Trump referred to the Democrat as "Lamb the sham" and said he'd vote with his party in Washington if he's elected. He said 25 per cent tariffs on steel imports will boost Pennsylvania's economy.
"They had state judges that are Democrats change your voting districts - what kind of stuff is that?" Most Republicans do, as well. He said the tariff might have had more of an impact if the announcement had come earlier, and workers could feel some of the benefits.
The race has become an unlikely battleground in a district the president won by over 19 percentage points in 2016. "And this guy can really help me", Trump said during the long, raucous rally in Moon Township, during which he trumpeted his recent move to impose new steel and aluminum tariffs on foreign trading partners and his plans to sit down with North Korea's leader.
"I don't think President Trump hurt Rick Saccone's chances tonight, but I'm not sure he helped", said Nathan Gonzales, an expert on Congressional elections, on Twitter. But the president was risking a second embarrassing defeat after backing Republican Roy Moore but watching the Alabama Senate hopeful lose in December.
The Pennsylvania special election is to replace Republican Rep. Tim Murphy, who resigned previous year amid revelations of an extramarital affair in which the anti-abortion lawmaker urged his mistress to get an abortion when he thought she was pregnant.
Saccone, a four-term Pennsylvania House of Representatives member and former diplomat to North Korea, is running against Democrat Conor Lamb.
"Southwestern Pennsylvania doesn't need another downtown Pittsburgh liberal in congress", Saccone said.
Saccone accepted the support, telling the enthusiastic audience: "The president's support is key to obtaining victory on March 13, and so is your support".
But for nearly all of his 75-minute performance in front of a raucous crowd packed into a hangar, it was in-his-element Trump, vintage 2016: rambling and fiery, boastful and jocular - the part of being president that he loves perhaps the most.
Trump also reprised his criticism of liberal Senator Elizabeth Warren, whom he mocks as "Pocahantas", and Senator Bernie Sanders, the far left independent who challenged Hillary Clinton for the Democratic nomination in 2016. "He's a sleeping son of a b--, I'll tell you".