The fate of the overall legislation remains uncertain, with Democrats all but sure to need Republican support for it to survive. Action would then shift to the GOP-run House, where leaders have shown no taste for expanding background checks for private purchases.
Earlier, Reid decided to exclude a proposed assault weapons ban from the bill, fearing it would sink the legislation, but will allow a vote on the plan as an amendment. The ban's sponsor, Sen. Dianne Feinstein, D-Calif., seems sure to lose due to opposition from Republicans and moderate Democrats.
An Associated Press-GfK poll showed more than 8 in 10 people support broadening the background check requirement to gun shows. Other surveys show similar overwhelming support.
Critics appeared unbowed. The NRA and others say criminals ignore background checks in getting guns illegally, and warn the expansion would lead to a federal registry of gun owners.
"We remain as committed as we have been to opposing gun bans," said Andrew Arulanandam, an NRA spokesman. He declined to comment on a potential compromise but said if the Senate considers Schumer's version of background checks, "we will do whatever we can to defeat it."
The NRA wants Congress to fund more armed guards at schools, step up prosecutions of people who file false gun applications and increase the background check system's access to state records of people with mental and other problems.
Sen. Charles Grassley of Iowa, top Republican on the Senate Judiciary Committee, said of Reid's announcement, "I don't know how the leader expects members to vote on an ever-changing piece of legislation that has yet to gain bipartisan support."
In a hint of possible movement, one option that Schumer, Manchin and Kirk are considering would require background checks and record-keeping for private sales at gun shows and commercial sales online. It would exclude in-person, noncommercial transactions between people who know each other. The idea was described by two lobbyists and Senate aide who spoke on condition of anonymity to discuss private talks.