After the 30 days have passed, Congress can go back and change the law with amendments - which is actually the more common way to do it - but D.C. Del. Eleanor Holmes Norton (D) said that's unlikely.
While it is always wise to be strategic on matters that come before Congress, I do not believe that a serious attempt to overturn the Council bill will be made or would be successful.Critics worry that the District's recognition of gay marriage could set a precedent across the country, given that the area is governed by Congress. If Congress explicitly approves gay marriage now, it would be harder for them to overturn state laws.