One group involved, the Coalition to Stop Gun Violence, ran print ads in North Dakota newspapers criticizing newly elected Democratic Sen. Heidi Heitkamp after she expressed doubts about Obama's proposals.
Activists also have identified a few Senate Republicans they hope to sway, including Mark Kirk of Illinois and Susan Collins of Maine. In the House, they're focused on 35 to 40 Republicans in suburban areas or districts carried by Obama, where voters might be more supportive of gun-control measures.
"We have a million grass-roots supporters who have sent almost 200,000 emails to Congress, tens of thousands of phone calls and are ready to go to town hall meetings and camp out if they have to," said Mark Glaze, director of the mayors group. He said of lawmakers: "In the end, I'm confident that enough of them will look past the NRA's $2,000 contribution and do what the public is demanding."
However, the NRA, with about 4 million members, has already activated its base, issuing a fiery appeal this week in which Executive Vice President Wayne LaPierre warned backers: "It's about banning your guns, PERIOD! . . . I warned you this day was coming, and now it's here. This is the fight of the century."
As publicity spreads about Obama's proposals, the NRA has been adding about 8,000 members a day, according to the group's president, David Keene. The NRA grades lawmakers on votes and has had apparent success in swaying congressional debates for years.
"We support the folks who've helped us in the past, and we remind them that we're also interested in what they do today and tomorrow," Keene said. "I'm convinced that, once this thing gets debated, the folks who've been with us in the past are probably going to be with us in the future."
Obama's call for an assault weapons ban is a particularly heavy lift, but backers are more optimistic about increased background checks, which were favored by 84 percent in an Associated Press-GfK poll this week.
Supporters hope those kinds of poll numbers will help move lawmakers to buck history -- and the NRA -- and vote in favor of gun-control bills.
"We definitely have our work cut out for us. The math's not with us right now, in terms of the votes," said Andy Pelosi, president of Gun Free Kids. "It's going to be difficult, but I am optimistic. I think the tone in the country is much, much different, and you can't underestimate that."