Think back on it... 2013 was very, very close and we also had the GOP House as a backstop.
Not only that, we had a functional NRA twisting arms on Capitol Hill, and months of build up with people flooding their Senators every day with calls, letters, email, etc. I remember calling my reps almost daily from for that whole time, from the start of the session until it went down in April.
If we lose GA it will be the same scenario as Clinton's first term... absolutely nothing to stop Biden from ramming through anything he wants until the 2022 elections start coming up and he has to be more accommodating. The only reason there was even a sunset clause was because a couple of GOPers insisted. Heck, George W promised to renew it when he ran for President... but 9/11 made that politically impossible.
Obama tried the same thing... Get gun control done right up front with his 'Iron River' meme about US guns ending up in Mexico. Then Fast & Furious got exposed and he had to back off. He couldn't do anything that would cost votes in 2010, and he still lost the House then, which made his plans almost impossible.
The simple fact is that Biden has already outlined his gun plans, starting with a new AWB as the centerpiece. If he has both chambers, he can pass anything he wants to.
I'm sure he'll start with the popular laws, like 'universal' background checks and a national red-flag law. Those will sail through and the media will give him all the cover he needs. Then, the next time an AR or AK is used in any kind of mass attack, it will be a full-court press on the AWB.
It worked for Clinton, and would have worked for Obama except for F&F blowing up. It will certainly work for Biden. Let's not forget, he was chairman of the Senate Judiciary committee responsible for AWB #1.
And as for support, most polls show about 60% of all Americans support a new AWB. It's almost 90% for 'universal' background checks, and around 80% for red flag laws. Sadly, even among gun owners there's a lot (~50%) of support for the last two.
That's why it's going to be so critical to win the GA Senate. Even if you hate one or both of the GOP candidates, hold your nose and vote. Otherwise that whole 'Dems don't actually pass gun control' meme will get stuffed down our throat sideways.
