Courts may ultimately block House attempts to get the tax returns if Mueller has already acquired them, investigated and explicitly cleared Trump.
(September 2017)
"Why Robert Mueller Probably Has Trump's Tax Returns"
https://www.usnews.com/news/national-news/articles/2017-09-18/robert-mueller-likely-has-donald-trumps-tax-returns
"IT NOW APPEARS LIKELY special counsel Robert Mueller has crossed what President Donald Trump has said is a clear red line by gaining access to the president's tax returns as part of a broadening investigation looking for links between Trump's business interests, his presidential campaign and Russia.
In fact, it seems almost certain the FBI–special counsel investigation has its hands on the president's tax returns.
"Mueller would be engaged in malpractice if he didn't" already have access to the president's tax returns, a member of Congress on one of the congressional committees looking at Russia's interference in the 2016 presidential election told me.
In fact, people familiar with the type of investigation that Mueller is now running signal the near-certainty that Mueller has access to the president's tax returns. The purpose would be to use the tax returns as a road map to investigate potential Russian financial influence within Trump Organization limited liability companies."