Democrats are planning to turn up the heat on the Republican tax law as they expand their power on Capitol Hill.

After flipping 40 House seats in November's midterm elections, Democrats will become the majority in the chamber next month — and they have key parts of the signature legislative achievement of President Donald Trump's administration in their crosshairs.

One emerging line of attack takes aim at public companies such as General Motors and AT&T that have laid off workers despite receiving big tax breaks under the new law.

Democrats are also promising changes to the new limit for households that deduct state and local taxes, which hits blue states particularly hard.

Massachusetts Rep. Richard Neal, incoming chairman of the tax-writing House Ways and Means Committee, has vowed to hold hearings on the sweeping $1.5 trillion legislation.

The tax law failed to resonate with voters during the midterm campaign, and now Democrats are looking to press the momentum against it. Activists are already mapping out the battleground. The Communications Workers of America on Monday called on Neal to put AT&T in the hot seat. The union estimated the company has received $3 billion in tax benefits and slammed its decision to lay off more than 7,000 workers this year.

The union is in the midst of contract negotiations with AT&T and represents roughly 100,000 employees. The company gave workers a $1,000 bonus shortly after the tax law passed last year, but CWA President Chris Shelton said promises of wage increases have not materialized.

Democrats "are going to shine the light of day on the large amounts of money that these companies receive by the tax cuts," Shelton said. "At the very least, I think we'll be able to find out more about where the money went."

GM has endured similar criticism after announcing plans last month to lay off 15,000 workers and shut down its iconic plant in Lordstown, Ohio. Three senior Democratic senators — Chris Van Hollen of Maryland, Amy Klobuchar of Minnesota and Tammy Duckworth of Illinois — are demanding that the automaker cancel planned stock buybacks as a result.

The senators also take issue with a provision in the law that reduces the rate that companies pay on foreign earnings. They argue that could incentivize businesses to shift production to other countries and pressed GM to disclose detailed financial information. So far, GM has booked $157 million in benefits from the tax law, according to its most recent earnings report.

"We need to understand how much GM can reduce its tax rate by expanding operations overseas while it closes factories here in the United States," the senators wrote in a letter to GM CEO Mary Barra on Friday. "This requires country-by-country information on GM's profits, taxes, employees, and tangible assets."