Updated: 3/13, noon

Since the Dec. 14 shooting that left 26 people dead at a Connecticut elementary school, at least 29 lobbyists have registered to influence Congress on gun-related issues, and several have been holding fundraisers for Sen. Susan Collins, a Maine Republican who is a potential swing vote in the legislative battle on Capitol Hill.

A review of lobbying disclosures, invitations on Political Party Time and ad buys on Sunlight's Political Ad Sleuth shows the influence machine gearing up as legislative activity does nationwide. Today, the Senate Judiciary Committee is scheduled to mark up federal gun control legislation, including a bill requiring background checks. On Monday, the Colorado Senate passed a package of gun control bills after an emotional debate; one that is now headed to the governor's desk will require gun purchasers to pay for their own background checks.

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Among companies and interest groups beefing up their presence in Washington on the gun issue is Dick's Sporting Goods, a national chain that sells guns. Last month, it hired the Monument Policy Group to represent its interests on the issue. One of Monument's lobbyists on the Dick's Sporting Goods account is Jane Alonso, a former legislative director for Collins.

On Wednesday, Alonso (along with many other lobbyists) will be hosting her second fundraiser in a little over two weeks for her former boss, a political centrist up for reelection next year. In late February, Alonso and her colleague Stewart Verdery, Jr., a former Senate Judiciary staffer, were among the hosts at another Collins event.

The senator, pictured at left, is providing crucial support for a bill that cracks down on "straw buyers" of firearms. She has also been the target of ads in her home state by groups on both sides of the gun debate: the NRA and Organizing for Action, the Obama campaign committee-turned-political nonprofit group.

Since the schoolhouse massacre at Newtown, Conn., a variety of pro-gun rights companies and advocacy organizations have hired lobbyists in favor of guns rights. Two groups are doing new hiring is in favor of stricter gun control: The Mayors Against Illegal Guns Action Fund, a nonprofit group of mayors led by New York City Mayor Michael Bloomberg, and the Brady Campaign Against Gun Violence.

The mayors only had one lobbying firm on staff late last year but have added at least three more in recent months. The latest came late last week when the mayors signed up seven lobbyists from the OB-C Group, a Washington firm founded by Democratic operative Lawrence F. O'Brien III and GOP lobbyist Nick Calio (Calio has since left to lead the trade association for U.S. airlines). Lately, there have been many new registrations each week on both sides of the debate.

In February, the Brady Campaign hired its first outside lobbyists since 2004, when Congress voted not to renew the assault weapons ban. The Brady Campaign hired seven lobbyists from the influential Democratic firm Elmendorf Ryan, led by Steve Elmendorf, the past chief of staff to former House Democratic Leader Dick Gephardt, D-Mo., and Jimmy Ryan, a former advisor to Senate Majority Leader Harry Reid, D-Nev. The Democratic firm also recently picked up the Entertainment Software Association, the trade group for the video game industry, which has been blamed by some for glorifying violence.

On the other side, the big daddies of gun rights -- the National Rifle Association and the Gun Owners of America, which together spent at least $4 million on lobbying in 2012 -- have gained some new company in the halls of Congress. The new groups lobbying against gun control include the National Association for Gun Rights, which has positioned itself to the right of the NRA, and TheTeaParty.net, a nonprofit that hired its first lobbyist to push firearms issues and other "conservative political values."

The National Association for Gun Rights is one of the hosts of a fundraiser for Sen. Rand Paul, R-Ky., next week. Despite his hardline conservative stance, Paul said in January that he would consider legislation on background checks.

Sunlight has asked Collins' and Paul's offices for their stances on the Senate bill enhancing background checks, the Protecting Responsible Gun Owners Act of 2013, but aides have not responded.

Congress is only one of the legislative venues where the gun battle is being fought this year. Sunlight's monitoring service Scout, which captures legislation introduced in both Congress and the 50 state legislatures, is tracking hundreds of bills introduced by lawmakers on both sides of the gun debate. They run the gamut from bills that would nullify any new federal gun control measures to one Illinois bill that would require a psychiatric evaluations before purchasing a gun.

There has been an ongoing lobbying campaign on both sides of the debate in Colorado, a much-watched political bellwether with a strong libertarian streak. The state has suffered two of the nation's deadliest mass shootings in two Denver suburbs: last July a gunman left 12 people dead and injured dozens more at a movie theater in Aurora; in 1999, two students at Columbine High School killed 12 classmates and a teacher. Bills to close a loophole that allows purchasers in private gun sales to avoid background checks and ban gun ownership by those convicted of domestic violence were among the bills the Democratic-controlled state Senate approved. Those two bills now will be debated in the state House, where Democrats also hold a majority.

The National Rifle Association has four active lobbyists in the Rocky Mountain State, according to lobbying records, and has maintained a lobbying presence for many years. Mayors Against Illegal Guns is the new group in town, hiring three lobbyists since last July. Two of them were hired just days before the fatal shooting in the Aurora movie theatre that left 12 people dead and injured dozens more. In January, the group also blitzed the state with television advertisements on guns, according to Political Ad Sleuth.

(Nancy Watzman contributed reporting. Photo credit: Talk Radio News Service)

Note, 3/13, noon: This post has been updated to reflect that the Brady Campaign to Prevent Gun Violence has also signed up new lobbyists, according to a registration filed with the Senate Tuesday by the firm Elmendorf Ryan.