Via email, Pat Egan emails his thoughts on that counterfactual question:
If the Democrats controlled the House right now, there’d be an assault weapons ban on the books in two months. But if there were an assault weapons ban on the books right now, the Democrats wouldn’t even control the Senate.
The logic: A Republican-controlled House will never pass an assault weapons ban. If the Democrats had passed a ban when they last controlled both chambers of Congress (2009-2010), it would have required the votes of Democratic senators from rural, pro-gun states including Jon Tester (MT), Claire McCaskill (MO), and Harry Reid (NV). Casting these votes would have made it much harder for these and other rural Dem senators to win the (often-narrow) election victories that have kept the Senate in Democratic hands.
A lot depends on whether everything else is equal in this counterfactual. Reid and McCaskill won convincingly enough—and against weak enough opponents—that I’m not sure their vote on an assault weapons ban alone would have cost them the election. (Unless such a vote produced somehow produced a better GOP challenger.) And would an assault weapons vote have mattered above and beyond other controversial roll call votes during that time, especially health care reform?
Tester did face a better opponent, but his race also wasn’t that close—he won by 4 points. But since our estimate of the penalty from supporting health care reform was almost 6 points, I don’t want to rule out the possibility that support for the assault weapons ban could have erased Tester’s margin of victory. But a 4-point penalty would still be fairly large, as judged by the effects of various roll call votes estimated in the literature.
Naturally, a lot in this counterfactual depends on the campaign activity on either side, gun rights or gun control. I’ll only note that Senate races see a lot more spending than the typical House race, making it harder for a single group—like the NRA or Handgun Control—to make as big a difference via campaign contributions or by spending independently.
Overall, I think that the Democrats would be more likely than not to control the Senate now had they passed an assault weapons ban when they were in the majority. That is a probabilistic statement, however, and perhaps others see Pat’s scenario as more likely.