The commerce clause. Article I, Section 8, Clause 3. Basically, if the government thinks that doing something to help private industry will somehow help the citizens of the nation overall, the government can do it.
There is plenty of corporate “welfare” in the history of our government. Remember bailing out GM in 2008? Remember the Federal Reserve bailing out banks with loans and guarantees? If the government (i.e., Congress) thinks that the handout will somehow benefit the nation’s citizens overall, it will do it.
Great example of this: The Federally chartered Union Pacific Railroad in the 1860’s. The act authorizing the UP gave every other section Federal land along the route to use for “railroad purposes”. The UP thought that means “anything we want to do”. This created the “checkerboard” patterm of the UP owning land in Wyoming and other states. The UP now has drilled natural gas wells in those sections and earned nice profits. Does this help the country? Sure, it is an energy source developed. Does it help the UP? Even moreso.