By Gram Slattery and David Shepardson

YORK, Pennsylvania (Reuters) -Republican presidential candidate Donald Trump pledged on Monday to rescind a Biden administration rule limiting power plant pollution, and reiterated a promise to block Nippon Steel's planned purchase of U.S. Steel.

In the most detailed economic remarks he has offered on the campaign trail so far, Trump also ratcheted up his pledge to radically limit foreign access to domestic markets, saying he would ensure the supply chain for essential goods would be 100% American if he wins the Nov. 5 election. He did not elaborate on how he would accomplish that goal.

The former president spoke to a crowd of supporters on a factory floor in the working-class city of York, Pennsylvania. The speech was the first stop in an intense tour of battleground states slated for this week, aimed at steering the spotlight away from the Democratic National Convention in Chicago and slowing the momentum of Vice President Kamala Harris, Trump's Democratic rival.

Some allies and advisers have pressed Trump to steer clear of personal attacks on Harris - particularly any broadsides related to race and gender - and instead focus more on policy issues.

He largely followed that advice on Monday. While Trump frequently denigrated Harris' policy stances, several of which he misrepresented, he attacked her in personal terms just once, when he insulted her laugh about 45 minutes into the hour-long speech.

Trump in February had signaled he would move to block the Nippon-U.S.Steel deal, a potential merger that has stirred anxiety among some unionized workers.

Shares in U.S. Steel had been lower and fell after Trump's planned remarks were reported - they were recently trading down 3.5%. President Joe Biden in March said it is vital for U.S. Steel "to remain an American steel company that is domestically owned and operated."

Trump also pledged to eliminate rules published by the Environmental Protection Agency in April, which limit air and water pollution from power plants and are designed to cut over 1 billion metric tons of greenhouse gas emissions by 2047. The electricity sector is responsible for nearly a quarter of U.S. greenhouse gas pollution, according to the EPA.

"It's a disaster for our country," Trump said. "Instead of shutting down power plants, we will open dozens and dozens more, and they'll happen fast."

Trump added in his remarks that his administration would bring more "advanced small, modular nuclear reactors" online and that he would invoke the Defense Production Act to ramp up production of "essential products."

Even as he pledged broad tax cuts for families, businesses, Social Security disbursements and tips, he said he would slash the deficit, a claim that good government experts have cast doubt on.

Pennsylvania has emerged as one of the most important states in the presidential election, with Harris and Trump effectively neck-and-neck, according to most opinion polls.