PITTSBURGH — President Joe Biden promised cheering unionized steelworkers on Wednesday that his administration would block the acquisition of U.S. Steel by a Japanese company, and he called for a tripling of tariffs on Chinese steel, seeking to use trade policy to win over working-class votes in Pennsylvania, an election-year battleground.

Biden said during a visit to the headquarters of the United Steelworkers that U.S. Steel “has been an iconic American company for more than a century and it should remain totally American.”

“American-owned, American operated by American union steelworkers — the best in the world — and that’s going to happen I promise you,” he said.

The Democrat’s administration is reviewing the proposed acquisition of U.S. Steel by Japan’s Nippon Steel. The president said last month he would oppose the deal, saying it was “vital for it to remain an American steel company that is domestically owned and operated.”

But in front of a pro-union audience, he went much further in promising to block the deal. “The backbone of America has a steel spine,” Biden said.

In another move that his administration argues can protect domestic steelworkers, Biden also announced that he will push for the higher tariffs on Chinese steel and aluminum, aiming to insulate American producers from a flood of cheap imports.

The moves reflected the intersection of Biden’s international trade policy with his reelection effort, although the White House insisted they were more about shielding American manufacturing from unfair trade practices overseas than firing up a union audience.

The current tariff rate is 7.5% for both steel and aluminum but could climb to 22.5% under Biden’s proposal. The president said he was asking his trade representative to seek the increase.

Liu Pengyu, a spokesman for the Chinese Embassy in Washington, said the “U.S. is making the same mistake again and again” by seeking increased tariffs. In a statement, he also dismissed those levies already in place as “the embodiment of unilateralism and protectionism of the U.S.”

Biden insisted that getting tougher on China was sound policy.

“I made sure that we had the most advanced technologies that we’ve developed and invented, and they can’t be sent to China because they’ll undermine our national security,” he said, adding that he had delivered a similar message to Chinese President Xi Jinping during previous conversations. Biden said he told Xi that “you’ll use them for all the wrong reasons, so you’re not going to get those advanced computer chips.”

The administration also promised to pursue investigations against countries and importers that try to saturate existing markets with Chinese steel.

“The president understands we must invest in American manufacturing. But we also have to protect those investments and those workers from unfair exports associated with China’s industrial overcapacity,” said White House national economic adviser Lael Brainard.