With the Senate voting comfortably Thursday to take up its most ambitious gun control legislation in nearly two decades, all eyes turn to the “open amendment process” that Majority Leader Harry Reid, D-Nev., has promised.

Senators who advocate gun control will try to add much tougher provisions to curb firearms and ammunition. But pro-gun senators will attempt to gut the legislation (S 649) with amendments backed by the National Rifle Association.

“I’ve been around these things,” Sen. Charles E. Schumer, D-N.Y., said Thursday. “The NRA will try to throw all kinds of amendments at us, and we’re going to have to work really hard to prevent them from being added to the bill.”

Here is an early list of five amendments to watch, including some that could turn what is now a sweeping gun control bill into one that significantly expands gun rights.

#1: Concealed Carry

Schumer issued an early warning Thursday on one particular amendment that he anticipates from his pro-gun counterparts: a plan to ensure that concealed carry handgun permits issued by one state are recognized by all others.

The proposal, known as “concealed carry reciprocity,” came within two votes of Senate passage as an amendment to defense authorization legislation in 2009. Schumer calls the proposal “pernicious.”

“If Wyoming has a concealed carry law, somebody could come from Wyoming to the big cities of New York or New Haven or Bridgeport and carry a concealed weapon, which is so against our way of life and the needs here in New York,” he said during an event with Connecticut’s two Democratic senators.

So far, no senator has pledged to offer the amendment in the current gun debate, but that is certain to change. Sen. Tom Coburn, R-Okla., on Thursday said he would “offer or support amendments to protect the Second Amendment rights of veterans and Americans who have concealed carry permits.”

Sen. John Thune, R-S.D., offered the amendment in 2009, when it attracted the support of all but two Republicans plus 20 Democrats, including Reid and 12 other Democrats who are still in the Senate.

#2: Mental-Health Records

A bipartisan and influential group of senators has signed on to a proposal that seeks to ensure that more records of those with mental-health problems are included in the National Instant Criminal Background Check System, which is used to determine whether people may buy guns.

The plan is co-sponsored by Republican Sens. Jeff Flake of Arizona and Lindsey Graham of South Carolina and conservative Democratic Sens. Mark Begich of Alaska and Mark Pryor of Arkansas.

But while it is supported by the NRA, the proposal is seen as a major threat by Mayors Against Illegal Guns, New York Mayor Michael Bloomberg’s gun control group.

“Though it purports to clarify and increase mental health record submissions to the gun background check system, [the proposal] instead would allow many more individuals who have been involuntarily committed or otherwise found to be seriously mentally ill to buy and possess guns,” Mark Glaze, the group’s director, said in a statement. “In fact, it weakens current law governing these cases.”

Graham intends to offer the proposal as an amendment, and the fact that conservatives and moderates in both parties have signed on suggests that it stands a strong chance of succeeding. Sen. Richard Blumenthal, a Connecticut Democrat who is one of the chamber’s strongest supporters of tougher gun laws, initially backed the plan but withdrew his endorsement this week.

#3: Armed Prosecutors

Senate Minority Whip John Cornyn, R-Texas, may offer another amendment that would expand gun rights, and his timing could make it difficult for Democrats to vote against it.

Following the recent killing of two state-level prosecutors in Texas — as well as the state prisons chief in Colorado and a sheriff in West Virginia — Cornyn has introduced legislation that would allow all judges, prosecutors and law enforcement officials at the state and federal levels to carry firearms into all federal facilities, according to a summary from his office.

The proposal also would allow prosecutors and law enforcement officers to “possess and obtain an ammunition magazine of any size.”

#4: High-Capacity Magazines

Blumenthal will offer an amendment that would ban the future production of ammunition magazines holding 10 rounds or more. The same provision is contained in the broader assault weapons ban sponsored by Sen. Dianne Feinstein, D-Calif., and Reid has guaranteed that both proposals will receive votes as amendments.

By stripping out the ammunition component, however, Democrats are telegraphing that they think it has a stronger chance of succeeding on its own. Reid voted against the original assault weapons ban in 1994, but this week he praised Blumenthal’s amendment as a “very forward-leaning proposal.”

Both proposals are highly unlikely to be adopted. But it will be worth watching how conservative Democrats who have criticized the assault weapons ban as too far-reaching will vote on the ammunition provision, which President Barack Obama has emphasized at nearly every turn.

#5: Background Checks

Without question, the linchpin of the gun legislation is the bipartisan proposal that Sens. Joe Manchin III, D-W.Va., and Patrick J. Toomey, R-Pa., unveiled this week to require background checks on all gun sales in commercial settings, including at gun shows and on the Internet.

Democrats are pinning their hopes on the compromise, which Reid has guaranteed will be the first amendment. Thursday’s lopsided 68-31 vote in favor of cloture suggests that some Republicans are, at the very least, interested in the plan.

But the amendment will be up against opposition from the NRA and will face competition from other GOP proposals on background checks, including one from Coburn and potentially another from Iowa Sen. Charles E. Grassley, the top Republican on the Senate Judiciary Committee.

Coburn said Thursday that he will offer an amendment “to replace the unworkable Manchin-Toomey [language] with a proposal that will protect Americans’ Second Amendment rights while giving law-abiding citizens the tools they need to make sure they aren’t transferring a firearm to someone who will be a threat to themselves or others.”

If one of the GOP alternatives is adopted in place of the Manchin-Toomey amendment, Democrats could rapidly fall off from supporting the underlying bill and the entire legislation could be in jeopardy.