|"What law says that the President has to show his tax returns?"|
None - it was common practice since Nixon. There's at least one state (NJ, I think) where a law is pending that will require the release of tax returns from all presidential candidates in order to get on the ballot in that state. That's what happens when you violate a tradition that people expect you to adhere to.
"What law says congress has the right to see anyone's tax return?"
26 USC 6103(f)(1), which says:
"Upon written request from the chairman of the Committee on Ways and Means of the House of Representatives, the chairman of the Committee on Finance of the Senate, or the chairman of the Joint Committee on Taxation, the Secretary shall furnish such committee with any return or return information specified in such request, except that any return or return information which can be associated with, or otherwise identify, directly or indirectly, a particular taxpayer shall be furnished to such committee only when sitting in closed executive session unless such taxpayer otherwise consents in writing to such disclosure."
Notice that it says Mnuchin, as Secretary, SHALL furnish - not MAY furnish only if he decides the request is appropriate. Mnuchin doesn't get a say in the matter.
"What info could there possibly be on his return that makes him ineligible to be President?"
That makes him ineligible to be President? None that I can think of. He's an American citizen over the age of 35, so he's eligible. That makes him subject to impeachment and removal from office? A lot of stuff, starting with tax fraud and financial fraud. If the House committee obtains his returns and matches them up against financial statements he supplied to banks or even his recent financial disclosure statements, and they don't match up - Houston, we have a problem...
"This is all just lefty smoke screen.
You do realize that don't you?"
Not at all. As someone who grew up in the NYC area, I already knew that Trump was shady, which is why I am so opposed to him. It has nothing to do with left-wing politics and everything to do with what's best for ths country - and he ain't it.