Democrats on Tuesday moved closer to a gun control vote in the US Senate, as Vice President Joe Biden slammed "mind-boggling" Republican threats to block the reform legislation following the Newtown massacre.
Senate Majority Leader Harry Reid expressed shock that 13 conservatives had lined up to prevent the vote, and he moved to overcome the tactic, known as a filibuster, by setting up a test vote for Thursday.
That procedural hurdle looked increasingly likely to be cleared, with several Republicans signaling they would defect from their party's hard line, and Democrats guardedly optimistic that a compromise on background checks for gun sales could be ironed out this week.
"We need to move to this legislation," Reid told reporters, weeks after the Judiciary Committee approved a three-part bill calling for background checks for all gun sales, felony penalties for gun trafficking, and new funding to boost school safety.
"It would be a real slap in the face to the American people not to do something" on the elements of the bill, which enjoy broad public support according to national polls.
Reid warned that inaction would show lawmakers were not committed to keep Americans safe from gun violence, notably after 20 children were killed along with six adults in Newtown, Connecticut last December.
"We are really failing. We need to do more," he said.
President Barack Obama, who visited Connecticut Monday, piled on the pressure by phoning Republican and Democratic senators Tuesday to discuss gun control measures, according to a White House official.
He faces resistance from Republicans in conservative-leaning states where hunting is prevalent, and where efforts to crack down on gun violence are seen by many voters as infringing on the constitutional right to bear arms.
Meanwhile in a concerted White House push, Biden warned that the world would be mystified if Senate Republicans balked at a gun vote.
"It appears that now not only are some of the senators not willing to stand and be counted, they're prepared to stop anybody from being able to be counted. I mean, it's almost mind-boggling," he said.
Biden, who served in the Senate for nearly 40 years, sounded exasperated at the thought that "the climax of this (Newtown) tragedy could be we're not even going to get a vote."
But by late Tuesday a vote was looking increasingly likely. At least 12 Republicans have told reporters they would like to have an open debate and vote on gun legislation, meaning Reid would likely have the 60 votes necessary to overcome a filibuster in the 100-seat Senate.
Biden said he spent two hours Tuesday with family members of those killed at Newtown's Sandy Hook Elementary School. Many of those relatives also lobbied senators.
"What are you going to say to those parents?" he asked of lawmakers. "Look them in the eye and tell them you concluded there is nothing you can do?"
White House calls for a new ban on assault rifles and limits on the size of fast-firing magazines look unlikely to pass Congress amid intense opposition from the gun lobby and most Republicans.
The best hope for reform now lies in a drive to expand background checks for all gun purchases, and Democrats are desperate for bipartisan backing, which would be crucial for passing the Republican-led House of Representatives.
Democrat Joe Manchin is deep in negotiations with Pennsylvania Republican Pat Toomey over new language in an amendment that would expand mandatory background checks to include sales at gun shows, which are currently exempt from such checks.
"We really are getting there," Manchin told reporters, adding that by Wednesday "we hope to be at a point to where we can finalize everything."
Added Democrat Chuck Schumer: "We're not there yet (but) we're closer than we've ever been."