WASHINGTON - Senate Democrats are scrambling to defeat a Republican-backed provision that would let gun owners carry concealed weapons across state lines, overriding the stricter laws of many jurisdictions.

Critics, including police organizations, big-city mayors and gun-victims groups, decried the provision, saying it would create "a new national lowest common denominator" for ownership of firearms. But twice this year, Republicans have rolled back restrictions on guns with substantial backing from moderate Democrats, many of them newly elected from Western states with strong Second Amendment traditions.

The latest measure, offered by Sen. John Thune (R., S.D.) as an amendment to the defense authorization bill, would let people carry concealed firearms across state lines if they "have a valid permit or if, under their state of residence ... are entitled to do so."

The amendment is opposed by 400 mayors, who outlined their objections in a letter to House Speaker Nancy Pelosi (D., Calif.) and Senate Majority Leader Harry Reid (D., Nev.). But Reid voted for both pro-gun measures earlier this year and is viewed as a likely "yes" on Thune's amendment.

Thune called his amendment a crime-prevention tool. Sen. Charles E. Schumer (D., N.Y.) has pledged a filibuster.