Sen. Kay Hagan (D-N.C.), who is up for reelection this year, works with her staff Wednesday in the Senate Reception Room. (Melina Mara/The Washington Post)

This was going to be a quiet week in the U.S. Senate, one during which the majority Democrats would take the opportunity to help a colleague facing a tough reelection race.

But what was supposed to be an innocuous election-year proposal, designed to draw considerable bipartisan support, has become the first big fight over gun control in Washington in more than a year — reopening old wounds for the very Democrats it was supposed to help.

The measure would expand access to hunting and fishing on federal lands. Sen. Kay Hagan (D-N.C.) is the lead sponsor. She faces one of the most challenging reelections this year, so she convinced 45 other senators — including 26 Republicans — that they should back the bill, making it one of the most popular legislative proposals debated by the Senate this year.

Even in a badly fractured chamber where bipartisan agreement is rare, passage of the Hagan bill should be a slam dunk.

But it’s now caught up in something else entirely. Both gun rights proponents and gun-control advocates are looking at the bill as a way to score political points by adding amendments.

“I want this debate. I want an opportunity to raise important issues about gun violence and gun safety in America,” Sen. Richard J. Durbin (D-Ill.) said Wednesday. He is seeking to stiffen penalties for illegal straw purchasers, or people who buy guns for others who plan to use the firearms to commit a crime.

On the other side, Sen. Rand Paul (R-Ky.) wants to overturn the District’s ban on large-capacity magazines and assault weapons and allow gun owners to carry firearms into post offices and other federal buildings and sites.

This political gun fight comes about 15 months after a bipartisan gun-control measure was defeated in April 2013 despite the December 2012 mass shooting at an elementary school in Newtown, Conn., that drew national outrage and calls for new legislation.

As the fighting continues, Senate Majority Leader Harry M. Reid (D-Nev.) began using procedural steps Wednesday to advance the hunting and fishing bill, leaving open a slim possibility that Democrats and Republicans will agree to votes on gun-related­ amendments and pass the bill. If such an agreement can’t be reached, the measure could be quickly blocked and defeated.

Sen. Lisa Murkowski (R-Alaska­), the hunting bill’s lead GOP sponsor, said she was hopeful that a deal could be reached, noting that both parties have worked together at other points this year to pass modest bipartisan measures. “I want to believe that we can actually legislate around here,” she said.

If Reid opens the door to gun-related amendments, Sen. Tom Coburn (R-Okla.) wants to amend the bill to limit when veterans would be restricted from owning firearms because of mental illness. And Sen. Ted Cruz (R-Tex.) wants to expand the right to interstate transport and the sale of ammunition and firearms — proposals that are similar to language included in the bipartisan gun-

control measure that failed last year.

Democrats have other ideas, too.

Sen. Richard Blumenthal (D-Conn.), whose state is still reeling from the Newtown shooting, said he was discussing plans with other Democrats. “We should not be, in effect, expanding the use of guns without addressing gun violence,” he told reporters Tuesday.

If approved, the proposals would be added to Hagan’s Bipartisan Sportsmen Act, a measure that would make it easier to hunt and fish on some federal lands, allow the construction of more public shooting ranges at national parks and wildlife refuges and make it easier to purchase federal permits to hunt ducks, geese and other waterfowl.

Hagan’s involvement is notable because she, along with Sens. Mark Begich (D-Alaska), Mary Landrieu (D-La.), Mark Pryor (D-Ark.), Jeanne Shaheen (D-N.H.) and Mark Udall (D-Colo.) have earned special attention from Reid as they run for reelection.

At several points this year, Reid has either carved out time for floor debate of bills sponsored by the senators or allowed them to lead debate on other bills designed to become campaign fodder — regardless of whether they ultimately pass.

When the Senate debated reforms to the national flood insurance program, Landrieu — who represents a state still rebuilding after Hurricane Katrina and other natural disasters — played a prominent role in floor debate even though she wasn’t the bill’s lead author. Same for Pryor, who was a major advocate for two veterans’ bills that were ultimately defeated. Shaheen and Sen. Rob Portman (R-Ohio) earned floor time to debate long-sought bipartisan fixes to the nation’s energy efficiency programs, but the measure was defeated in a similar dispute with Republicans over amendments.

This week is Hagan’s turn, and she touted the bipartisan spirit of her bill Monday night as the Senate began to debate the bill.

“I recognize members on both sides of the aisle have ideas for how to strengthen our bill,” she said as she invited colleagues to propose their amendments.

Paul’s proposal won a swift rebuke Wednesday from Eleanor Holmes Norton (D), the District’s congressional delegate. She called Paul “the tea party standard-bearer” who “seems to leave his small government, libertarian views at the District line when it comes to the District of Columbia.”

The other GOP proposals probably would spell trouble for Hagan and the other Democrats facing reelection. In the past few years, several of them have been the arbiters of whether gun legislation passes or fails in the Senate. Begich and Pryor, for example, were among those who voted against last year’s gun-control bill.

Suddenly voting to loosen gun restrictions would do little to help their campaigns. Pryor had a C-minus rating from the National Rifle Association, a measure of how supportive he is of its causes and issues, as of December 2012. Gun Owners of America, another gun rights organization, gave him an F. Hagan has an F rating from both groups.

The NRA has given Landrieu and Udall C ratings under its scoring system. Begich is an outlier, with an A rating from the NRA. Landrieu’s and Udall’s ratings are a concern for both senators because both come from states with large numbers of hunters.

Gun-control groups — such as Mayors Against Illegal Guns, the group founded by Michael R. Bloomberg, the former New York mayor — lack such scoring systems but have spent money for and against some Democrats. The group has run ads against Pryor, Landrieu and Hagan. But other gun-control groups have backed several of these senators in 2014. Americans for Responsible Solutions, the organization run by former Rep. Gabrielle Giffords (D-Ariz.), is supporting Hagan’s and Udall’s reelections.

Aides to Hagan held out hope this week that her bill will advance, noting that 40 hunting, fishing or outdoors groups, ranging from the National Wildlife Federation to the Campfire Club of America, support the measure.

But the Humane Society is a vocal opponent of the legislation, in part because it would allow Americans who hunt for polar bears in Canada to import the trophy heads — a long-sought change in federal law among some hunters. The group, much like the gun lobby, described the bill as a political stunt.

“This is Congress as its worst,” the group said in a statement, faulting Democrats for giving Hagan, Pryor and the other vulnerable Democrats “a political talking point as they campaign in the rural areas of their states.”

Mike DeBonis and Jaime Fuller contributed to this report.