By Nandita Bose and Gabriella Borter

PITTSBURGH, Pa. (Reuters) -U.S. Vice President Kamala Harris said on Wednesday she would offer tax credits to domestic manufacturers and invest in sectors that will "define the next century", as she detailed her economic plan to boost the U.S. middle class.

Speaking at the Economic Club of Pittsburgh in the battleground state of Pennsylvania, the Democratic candidate in the Nov. 5 presidential election said she would give tax credits to U.S. manufacturers for retooling or rebuilding existing factories and expanding "good union jobs," and double the number of registered apprenticeships during her first term.

Harris also promised new investments in industries like bio-manufacturing, aerospace, artificial intelligence and clean energy.

Harris' speech, which lasted just under 40 minutes, did not detail how these policies would work. She highlighted her upbringing by a single mother, in contrast with Trump, the wealthy son of a New York real estate developer.

"I have pledged that building a strong middle class will be the defining goal of my presidency," Harris said, adding that she sees the election as a moment of choice between two "fundamentally different" visions of the U.S. economy held by her and her Republican opponent Donald Trump.

The vice president and Trump are focusing their campaign messaging on the economy, which Reuters/Ipsos polling shows is voters' top concern, as the election approaches.

The divide between rich and poor has grown in recent decades. The share of American households in the middle class, defined as those with two-thirds to double that of median household income, has dropped from around 62% in 1970 to 51% in 2023, Pew Research shows. These households' income has also not grown as fast as those in the top tier.

Trump, Harris said, is "only interested in making life better for himself and people like himself, the wealthiest of Americans."

She said she was committed to working with the private sector and entrepreneurs to help grow the middle class. She told the audience that she is "a capitalist" who believes in "free and fair markets," and described her policies as pragmatic rather than rooted in ideology.

Harris in recent months has blunted Trump's advantage on the economy, with a Reuters/Ipsos poll published on Tuesday showing the Republican candidate with a marginal advantage of 2 percentage points on "the economy, unemployment and jobs," down from an 11-point lead in late July.

Trump discussed his economic plan in North Carolina on Wednesday and said Harris' role as vice president gave her the chance now to improve the economic record of the Biden administration.

"Families are suffering now. So if she has a plan, she should stop grandstanding and do it," he said.While Trump has proposed across-the-board tariffs on foreign-made goods - a proposal backed by a slim majority of voters - Harris is focusing on providing incentives for businesses to keep their operations in the U.S.

Boosting American manufacturing in industries such as semiconductors and bringing back jobs that have moved overseas in recent decades have also been major goals for Biden. The Infrastructure Investment and Jobs Act, the CHIPS and Science Act, and the Inflation Reduction Act - all passed in 2021 and 2022 - fund a range of subsidies and tax incentives that encourage companies to place projects in disadvantaged regions.