After Ukraine and the Russia-hoax, it seems likely the Democrats in the U.S. House's quest for Donald Trump’s tax returns is just the latest effort to damage the President politically.  During oral argument over President Trump’s tax returns, USA Today sums up that point as follows:

On the other hand, all the conservatives and some liberal justices wondered whether the subpoenas go too far in seeking a decade of private data involving not only the president but members of his family. They suggested such extensive probing could harass and distract both Trump and future presidents.

. . .

“How can we both protect the House’s interest in obtaining information it needs to legislate but also protect the presidency?" Kavanaugh asked House general counsel Douglas Letter. . . .

But even liberal Associate Justice Stephen Breyer noted the subpoenas "go way, way beyond tax returns," a concern voiced by several conservative colleagues.