If the amendment passes, Bloomberg said, "states will have no ability to set their own gun laws – whatever they think is appropriate in regards to carrying concealed weapons."
He said that because states like New York would be forced to recognize concealed weapons permits from states with less stringent requirements to obtain permits, "the lowest common denominator would become the de facto common denominator."
He went on to call the amendment "anti-police" and "pro-gun trafficker" and said the practical effect would be to "put your life and your families lives are at stake."
The New York City mayor complained about efforts to attach the amendment to the defense appropriations bill in what he called a "time honored trick" to get it through the Senate. He said backers of the bill are "using our young men and women who are serving oversees" as pawns "to take away our safety from local criminals."
Bloomberg is among them more than 450 mayors who signed a full-page ad in USA today opposing the amendment, which Sen. Charles Schumer and families of the Virginia Tech shooting victims spoke out against Monday.
Bloomberg was joined on the call by Philadelphia mayor Michael Nutter, Jacksonville Mayor John Peyton and Milwaukee Mayor Tom Barrett. They complained that the Senate was rushing the amendment to a vote without debate and argued that it would never survive as a stand-alone bill.
According to Politico's Glenn Thrush, Senate Majority Whip Dick Durbin is working to win over six to eight moderate Democrats in order to kill the amendment. Senate Majority Leader Harry Reid is likely to back it.
"Three pro-gun measures have passed the Senate this year with the support of conservative and moderate Democrats," Thrush reports. "But Durbin said the Thune amendment is so radical, he may be able to peel off Dems who previously voted with the NRA."
Bloomberg said the amendment flies in the face of the National Rifle Association's longstanding argument that states should make their own gun laws. The NRA supports the amendment.
The New York City mayor said the effect of its passage would be to "put an awful lot more guns on the street" and "make it much more difficult for police departments across the country."
On the conference call, the mayors pointed to relatively lax requirements in other states to secure permits. They said that in Mississippi members of the Ku Klux Klan can get a permit, while in Texas and Alaska people who have been convicted of serious misdemeanors can secure them.
Nutter said the legislation would be "an incredible boon to illegal gun traffickers" because it would make it easier to transfer illegal weapons across state lines without being caught. Barrett said it represented "an attempt to essentially ignore the decisions made by state legislatures throughout this country."
"If this issue is so important, let's have the hearings on it," he said, complaining of the move to attach the amendment "as a rider to a must-pass bill."
Thune maintains that the amendment would "help in reducing crime by providing reciprocity for the carrying of concealed firearms."
"My legislation enables citizens to protect themselves while respecting individual state firearms laws," he said.