CNN  — 

President Donald Trump’s tariff announcement could send a direct signal to workers in the Rust Belt that his policies are looking out for them after decades of manufacturing employment decline.

Jobs in the steel and aluminum industries are concentrated in the Midwest swing stages that delivered Trump the election in 2016, according to census data. Many of these were the states that stunned the country when non-college-educated whites who had voted for President Barack Obama swung toward Trump.

Trump’s support in these areas has waned, however, and will be critical to win again as he gears up for re-election in 2020.

Many mills are in more reliably Republican states.

The US county with the most metal manufacturing employment – which is mostly composed of steel and aluminum manufacturing – is Lake County, Indiana. Indiana is home to the nation’s largest steel mill, Gary Works, in the city of Gary. The county with the third-most is also in Indiana, Porter County.

Ohio, Michigan and Pennsylvania are also home to counties in the top 10 of metal manufacturing employment, according to the census. Gallup found that approval for Trump has dipped below 50% in these states, which were crucial to his election. According to an analysis by CNN Senior Political Analyst Ron Brownstein, Trump’s approval among blue-collar women has declined in some of these states more than almost anywhere else since the election.

Notably, while many of these counties are in states that went for Trump, the counties themselves went for Hillary Clinton.

Cook County, Illinois, which includes Chicago, has the second-highest metal manufacturing employment, and it supported Clinton by over 1 million votes. Lake County in Indiana supported Clinton by over 40,000 votes. Four of the five counties with the highest metal manufacturing went for Clinton.

As CNN Money has documented, many economists are dubious that Trump’s invitation of a trade war will ultimately help the economy.