TRUMP’s TAX NO-RETURN

It should be obvious, that if Trump had nothing to hide, he wouldn’t fight so fiercely to keep his tax returns from going public. So, obviously having something to hide, he fights against exposure that could be humiliating, even incriminating. We don’t know. Yet.
But why the fierce support from the Republican front? Wouldn’t it be of national interest, possibly for security reasons, to know what he is trying so hard to hide? What happened to the 3-way separation of governmental power, the checks and balances? Aren’t congress members elected to support the interests of the people, and the security of the nation, more so than to protect a president who has been very secretive about what he has done, – and who keeps lying about it? And then we have AG William Barr, who cooked up a job application that would please Donald Trump, claiming that the Mueller investigation was a hoax, – without having any idea of what they were working on! And he got Congressional support?!