WASHINGTON - Another day, another contentious vote in the Senate.

An amendment to let gun owners bring weapons across state lines further complicates senators' consideration of the $680 billion defense authorization bill. (Getty Images) A vote is expected today on a proposal that would allow certain gunowners to bring their weapons across state lines.

The amendment would let people with concealed weapons permits carry their guns into other states as long as they follow that state's laws about where concealed weapons are permissible.

Just two states would not be part of the plan: Illinois and Wisconsin do not issue any conceal and carry permits so the amendment wouldn't affect them.

"Law-abiding South Dakotans should be able to exercise the right to bear arms in states with similar regulations on concealed firearms," the amendment's author, Sen. John Thune, R-S.D., said in a statement released Monday. "My legislation enables citizens to protect themselves while respecting individual state firearms laws."

As the chamber slogs through a massive $680 billion defense authorization bill, senators are considering a variety of amendments to the measure -- some of which have more to do with Defense Department spending than others.

On Tuesday, lawmakers removed controversial funding from the bill for seven expensive F-22 fighter jets. Today they turn their attention to questions raised by the right to bear arms.

But senators including Democrats Arlen Specter of Pennsylvania and Charles Schumer of New York, oppose Thune's effort.

"From my own experience growing up in Kansas and being District Attorney of Philadelphia, I know states need to prescribe their own rules for carrying a concealed deadly weapon," Specter said Tuesday. "This is the essence of federalism.

"My vote against the Thune Amendment will not limit the constitutional rights of hunters and gun owners," he said. "Pennsylvania already recognizes concealed carry permits from 24 other states where their laws are similar."

Schumer said he worries that states with stricter gun laws would be trumped by those with more lax requirements if the amendment becomes law.

"Each state has carefully crafted its concealed-carry laws in the way that makes the most sense to protect its citizens," Schumer said. "Clearly, large, urban areas merit a different standard than rural areas. To gut the ability of local police and sheriffs to determine who should be able to carry a concealed weapon makes no sense."

Still, the legislative climate may be ripe for the proposal to advance. Many new Democrats from conservative states are supportive of gun rights. Even Senate Majority Leader Harry Reid, D-Nev., has said he will vote in favor of the amendment.

Protecting Right to Self-Defense or Boon for Killers?

The vote expected today is yet another gun debate that extends beyond the walls of the U.S. Senate. More than 400 mayors have taken out full page ads in newspapers nationwide opposing the amendment. And families affected by the April 2007 shootings at Virginia Tech ran a full-page ad in Monday's Richmond Times-Dispatch urging Virginia Sens. Jim Webb and Mark Warner not to follow Thune's lead.

"Virginia has seen the worst consequences of guns falling into the wrong hands," the ad from Virginia Tech families says.

"But Congress is now moving a bill that would make Virginia's requirements meaningless by forcing our law enforcement to honor permits from states with weaker rules," it says. "That means that non-Virginians with concealed carry permits issued by any other state would be able to carry concealed handguns into our cities and towns, from Richmond to Norfolk and from Arlington to Blacksburg."

But pressure to support the gun amendment comes from another corner of the state. Advocates of Thune's amendment at the National Rifle Association headquarters in Fairfax, Va., are asking members to convince senators of the opposite stance, saying, "Now is the time for Congress to recognize that the right to self-defense does not end at state lines."

At the Brady Campaign to Prevent Gun Violence, those fighting for stricter gun laws said three recent shootings by permit holders are reason enough to reject Thune's proposal.

In February, four people died in update New York; in March, a man killed 11 people in a small Alabama town, and in April, three police officers died in Pittsburgh.

"It is an outrage that in a year thus far defined by gun violence -- from massacres, to the murder of police, to hate crimes -- the U.S. Senate is preparing to consider an amendment that would dramatically weaken federal and state gun laws," the Brady Campaign to Prevent Gun Violence said in a joint statement with Coalition to Stop Gun Violence, Freedom States Alliance, Legal Community Against Violence, States United to Prevent Gun Violence, and Violence Policy Center.

"The practical effect of the amendment would be to reduce concealed carry permit regulations to the lowest common denominator," the groups said.

Other gun proposals have also recently stirred up debate on Capitol Hill.

Republicans were able to stall a bill to give Washington, D.C., a vote in the House by inserting an amendment that would have taken away the city council's right to pass any gun restrictions.

And a provision to allow loaded guns in national parks if allowed by state law was attached to a credit card reform bill.

The gun amendment needs 60 votes in its favor to avoid a potential filibuster from Democrats who oppose the gun provision.

ABC News' Z. Byron Wolf, Tom Shine and The Associated Press contributed to this report.