U.S. President Donald J. Trump speaks during a "MAGA" rally at the Williamsport Regional Airport on May 20, 2019 in Montoursville, Pennsylvania.

U.S. President Donald Trump, facing a potentially difficult path to winning a second term in November 2020, on Monday told supporters in Pennsylvania that his trade war had strengthened the battleground state's steel industry and jobs.

Although Trump does not launch his re-election bid officially until next month, his appearance at a raucous rally in an airport hangar in northeastern Pennsylvania, using Air Force One as a backdrop, had the hallmarks of a campaign event.

He took aim at Democratic front-runner and former Vice President Joe Biden. "Sleepy Joe said that he's running to 'save the world.' ... He's going to save every country but ours," Trump said.

The president previewed the arguments he will make to voters in the 2020 election, crediting his trade stance with helping the U.S. economy.

"When you have the best employment numbers in history, when you have the best unemployment numbers in history, when you have the best economy probably that we've ever had, I don't know - how the hell do you lose this election, right?," he said.

Trump has waged a high-stakes trade dispute with China, and tariffs imposed by both countries on a range of goods have raised fears of a global economic slowdown.

Trump last year also imposed tariffs on steel and aluminum.

He claims that the move saved and created jobs at U.S. mills as well as spurred investment such as by U.S. Steel in Pennsylvania. Many economists say that those benefits are outweighed by higher costs to U.S. companies and consumers.

Trump was stumping for a special House of Representatives election in Pennsylvania, one of three "Rust Belt" states he won in 2016 with votes from white, blue-collar voters who had previously voted Democratic.

His campaign sees the state as key to keeping control of the White House, along with Michigan and Wisconsin. A Quinnipiac University poll last week showed the president trailing the main Democratic contenders in Pennsylvania in particular.

"I'll be seeing a lot of you over the next year," Trump said. "Gotta win this state."