Washington (CNN) - The Democrat who worked to expand background checks in the Senate told a group of reporters Thursday that the National Rifle Association's "scoring system," in which the group grades members of Congress, combined with members' fear of being seen as flip-floppers, derailed the measure.
Sen. Joe Manchin, D-West Virginia, who worked alongside Sen. Pat Toomey, R-Pennsylvania, said the NRA's decision to include votes on the bipartisan amendment in its important rankings dissuaded members in pro-gun states from stepping up. "If they hadn't scored it, we'd have gotten 70 votes," he said. "I predict 70 votes without a scoring." He also vowed to bring the measure back at some point for a vote.
The Manchin-Toomey measure got 56 votes on Wednesday – four short of the 60 needed to go to a final vote.
In addition, Manchin told a breakfast sponsored by the Wall Street Journal that senators were afraid of all the tough votes–and positions–they will be asked to take. With the Senate about to consider immigration reform–and with same-sex marriage becoming more of a political football–he said his colleagues were unwilling to become too controversial by changing positions, especially on guns.
"So a lot of them are saying, listen, I had a position on the gay issue, and I've always stated that position…and I changed," he explained. "Now, you've always been a strong defender, an NRA A-rated member. Now you voted for something, they scored against you, and you're not. So now you've changed on that too. Have you really? Are you still the same person that we sent? Now, how much energy does that take to defend?"
Manchin also claimed that it was "disingenuous" of the NRA to oppose background checks, accusing the gun rights lobbying group of distorting the components of his bill, such as when a check is required for an internet gun sale.
"They're hanging on one thread – one little thread, and that's internet sales," he said. "Now, if you have a loving relationship with your family member and your best friend, and you've got to sell your gun on the internet, you better check that relationship. That's how far – you see how much crap that is? That's how far they went with that. That's just disingenuous. And I've told them that. I said that's just a bunch of bulls–t."
The NRA did not respond to a request for a comment.
Meanwhile the group founded by former congresswoman Gabby Giffords and her husband, Mark Kelly, are vowing to continue pressing members of Congress to push for expanded background checks. Giffords, who was wounded in a January 2011 shooting in Tucson, and Kelly are both gun owners and have been defenders of the Second Amendment in the past. But they also say there needs to be "common sense" measures to prevent more gun violence.
Kelly told a Washington news conference Thursday their group, Americans for Responsible Solutions, will soon launch television commercials praising some of the senators who voted for the Manchin-Toomey measure: Democrats from "red states," such as Kay Hagan of North Carolina and Mary Landrieu of Louisiana, as well as Republican Sens. Susan Collins of Maine and John McCain of Arizona.
"We are not deterred by actions of a small group of senators" who did not listen to the majority of the public, Kelly said. He added: "That is unacceptable."
He claimed some of the senators who ended up voting against the proposal did so out of "fear of the gun lobby."
Kelly said his group will highlight to constituents of members of both parties how they did not support the proposal and said it is also considering running ads against some of them.
On Thursday following an interview his friend and fellow Arizonan, Sen. Jeff Flake, gave talking about his no vote, saying he wanted to keep guns out of the hands of criminals and those with mental illness, Kelly tweeted him: "@JeffFlake I'm confused, friend. You had that chance yesterday. Want to rethink and join me and Gabby in making Arizona safer?"
Flake's office did not respond to a request for a comment.
Earlier he told reporters: "If I were beholden" to the NRA, "I wouldn't have voted" last week to allow debate on the gun bill to proceed. "That's the beauty of a six year term."
He added: "What has been a little upsetting is to hear people try to maintain that we were just caving to pressure, and discounting any issues that we had with the legislation and with the language. And that's not right."
Following Wednesday's vote, Giffords and Kelly have said ultimately the make-up of Congress may be the only way to get some reforms passed. "We are going to have to change" who is in Congress, Kelly said at the news conference.
Organizers of their group, which is three months old, said it has 300,000 members now and has raised millions of dollars so far. They vowed to have a million followers within weeks.
Giffords sent an e-mail fundraising pitch to supporters Thursday, saying "since last night, well over ten thousand individuals have stepped up and contributed to our organization…to hold the Senators who opposed background checks accountable for their vote. I'm not going to stop fighting."
– CNN's Todd Sperry and Rachel Streitfeld contributed to this story.