WASHINGTON (CNN) - The hotly debated issue of gun control took center stage on Capital Hill Tuesday as key Senate Democrats bickered over a controversial proposal that would allow people to carry concealed weapons across state lines.
The measure, drafted by South Dakota Republican Sen. John Thune, would require each of the 48 states that currently allow concealed firearms to honor permits issued in other states.
Supporters of the measure argue it would help deter criminals; opponents claim it would endanger innocent people by effectively forcing most of the country to conform to regulations in states with the loosest gun ownership standards.
The issue has blurred Capitol Hill's usual partisan lines. Senate Majority Leader Harry Reid, D-Nevada, is one of several Southern and Western Democrats supporting the measure. Others Democrats oppose it.
The full Senate is set to vote on it Wednesday in the form of an amendment to a larger defense appropriations bill.
"This amendment is reckless (and) irresponsible," Sen. Dianne Feinstein, D-California, said at a news conference Tuesday.
"I believe it's going to lead to more weapons and violence on the streets of our nation. ... I have no doubt that if the amendment (passes), people will die because of it."
Feinstein was joined by, among others, Sen. Chuck Schumer, D-New York, who argued the measure is proof that Washington's potent gun lobby has gone "a bridge too far."
"We know that the gun lobby is strong," Schumer said. But this proposal "directly threatens the safety of Americans, particularly in urban and suburban areas."
Schumer and Feinstein argued that Thune's proposal had turned the traditional Republican emphasis on states' rights on its head.
"The great irony of this amendment is that the pro-gun lobby is doing a 180-degree turn to impose out-of-state standards on other states," Schumer said. "This proposal would trample other states' rights."
"The very people who treasure states' rights use it when it's convenient for their cause," Feinstein added.
Sen. Max Baucus, D-Montana, was one of several Western Democrats who cited a constitutional right to bear arms in announcing support for the proposal.
"Montanans who have concealed-weapon permits shouldn't have their basic Second Amendment rights limited just because they travel across state lines," Baucus said in a written statement.
This legislation "expands gun rights for folks who follow the law."
For his part, Thune predicted that the measure, if passed, would save lives.
"Since criminals are unable to tell who is and who is not carrying a firearm ... they are less likely to commit crimes when they fear that they may come in direct contact with an individual who is armed," he argued.
For gun control advocates, the fight over Thune's proposal is the latest in a series of unexpected setbacks since the start of the Obama administration.
In May, President Barack Obama signed a credit card bill that included a provision allowing people to carry guns in national parks. Another bill that would have given the District of Columbia's representative in Congress full voting rights stalled earlier in the year after Senate Republicans attached a provision that would have eased tight gun controls in the district.
–CNN's Greg Clary and Brianna Keilar contributed to this report