Nothing.
But what makes you think that congress may only pass constitutional laws?
They are absolutely free to pass any law they want whether constitutional or not. They aren't supposed to pass unconstitutional laws but they do it quite often.
No, they're not.
"The Congress shall have Power ... to make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof."
"This Constitution, and the Laws of the United States which shall be made in Pursuance thereof ... shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the Contrary notwithstanding."
"The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people."
http://www.usconstitution.net/const.html
The Constitution locks Congress into making only laws to carry out enumerated constitutional powers. Anything outside of those powers is reserved to The People and/or the States. So, even though Congress routinely passes unconstitutional laws, and SCOTUS routinely finds ways to declare them legal, they are not constitutional.
This is where someone like you needs to stand up and challenge the law. This is why we don't have only one branch of government. This is exactly why we have checks and balances built into our government.
Does that answer your question without actually answering it?
Maybe I should start buying lottery tickets so that I could get the money necessary to challenge the numerous unconstitutional laws.
SCOTUS (or maybe Congress, I'm not sure) has set up a huge roadblock to challenging laws called "standing." I can explain it if you wish. It would keep me from challenging DOMA and many other unconstitutional laws (maybe even the "Patriot" act) no matter how much money I spent.
As for checks and balances, the federal branches were set up as checks and balances, but they've broken down. The legislative and executive branches are dependent on and indebted to the same corporate sponsors, and they only hire justices who are sympathetic to their view of how things ought to be, rather than the actual Rule of Law.