Congress has no constitutional authority to pass a law legalizing same sex marriage other than pertaining to parts of marriage that affects the federal branch such as federal taxes, immigration purposes.

Marriage is a state right unless their laws conflict with the constitution.

The recent bill they are wanting to pass to protect marriage in congress contains a part which obligates states to recognize same sex marriages from other state. If Obergefell is overturned that part of the law will likely be overturned too.