Well, with any luck he won't HAVE TO veto it; The "assault weapon" ban sunsets unless a law is enacted to extend it, and it has always been easier to block the passage of new laws through Congress, than to enact new laws. We don't need to overcome a veto, we need to win a filibuster in the Senate; 41 votes will do that.
The real challenge won't be fighting off the extention, it will be knowing when to fight. It's not like they're going to openly bring forward a single issue bill extending the ban; Not unless Congress and the White house fall into Democratic hands. They'll sneak a provision into a conference bill in the dead of the night, and not tell anybody about it until after it's been voted on. That's how they got the Lautenburg amendment enacted, after all.
There's only two ways to stop that: Either get pro-gun leadership in Congress, (The current Republican leaders obviously doesn't count!) or get the rules in Congress changed so as to make sneaking things into bills much harder. Actually, you'd probably want to do both.
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Sic semper tyrannis!
