This post has been updated with comments from Democrat Katie McGinty's Senate campaign. 

The 26-hour sit-in on the House floor this summer to demand a vote on gun control was a near-perfect display of the movement's current partisan makeup: House Democrats launched it. Republicans blasted it.

But that's exactly why a Democratic-led gun control group is choosing Republicans over Democrats in at least two key Senate races. In endorsing Sens. Pat Toomey (Pa.) and Mark Kirk (Ill.), Americans for Responsible Solutions is playing the long game to try to win over Republicans to their cause — even if it risks hurting some Democrats in the interim.

Former congresswoman Gabby Giffords (D-Ariz.), who founded the centrist gun control group after she was shot and nearly killed in 2011, endorsed Toomey and Kirk in a CNN op-ed on Monday. More endorsements are expected this fall.

And former New York mayor Michael Bloomberg, an independent and Hillary Clinton supporter, endorsed Toomey earlier this month, citing the senator's leading role on a background check bill after the 2012 Sandy Hook massacre. Bloomberg's super PAC released an ad with the daughter of the Sandy Hook principal saying she's "grateful" for Toomey:

In Pennsylvania, grass-roots groups affiliated with Bloomberg's Everytown for Gun Safety group are actively campaigning for Toomey over his Democratic challenger, Katie McGinty. The message is clear: The gun control movement has the back of Republicans who cross party lines for their cause.

This long-term plan to win over Republicans is not without some short-term risk. For one, Toomey isn't a perfect model of consistency on gun control. He actually voted with the National Rifle Association this summer on a proposal to prevent suspected terrorists from getting guns, proposal Democrats called toothless. On the campaign trail this summer, Toomey touted his "perfect track record with the NRA."

But more importantly, Democrats traditionally champion gun control, and we're less than three months from an election where Democrats have a solid chance to take back control of the Senate and possibly even the House of Representatives. If you're a gun control group, why do anything to risk a political outcome balanced in your favor? (Toomey's race in particular will be fought tooth and nail.)

"A Senate controlled by Mitch McConnell isn't going to bring up gun safety laws, regardless of what a few members of his caucus think," said Sean Coit, communications director for McGinty's campaign, which was not happy with being skipped over for this endorsement.

One Democratic strategist not affiliated with Americans for Responsible Solutions says gun control groups have no choice but to try to win over Republicans: Even under the rosiest of November scenarios for Democrats, they won't have the numbers to pass gun control legislation on their own. Gun control groups need people like Toomey and Kirk on their team.

Actually, they need a lot more than just Toomey and Kirk. And they could be close to winning others.

After the June Orlando massacre, the door opened a crack for bipartisan movement on gun control. Eight Senate Republicans voted for a compromise bill to prevent people on the FBI's various terrorist watch lists from legally being able to buy guns — including Sen. Kelly Ayotte (R-N.H.), who voted against a similar proposal in December. She's in a tough reelection too and is running ads championing her vote for a background check bill (though Americans for Responsible Solutions is running counter-ads claiming she didn't vote for the right background check bill).

Ultimately, the terror watch list compromise got 52 votes; a majority, yes, but shy of the 60 needed to advance.

Groups like Americans for Responsible Solutions are trying to make sure the door Orlando opened to bipartisan gun control legislation doesn't shut entirely. And the best way to do that is to reward Republicans who voted with them, even if it comes at the expense of Democrats.

The decision to endorse Toomey and Kirk is also a win-win for these groups' agendas. Yes, it upsets some Senate Democrats. But if Toomey and Kirk win, the gun control movement has strengthened a politically useful partnership. If Toomey and Kirk lose, the gun control movement still gets Democratic candidates who support their cause. In her op-ed announcing the endorsements, Giffords called the decision to support Toomey and Kirk "difficult."

If Toomey wins his deadlocked race against McGinty, you could argue it was the gun control groups' support that helped him over the top.

Toomey's fate would be an attractive one to, say, a Republican lawmaker in a swing district in Ohio or New Hampshire whose constituents support gun control but whose party at-large doesn't.

For a generation, the National Rifle Association has put significant amount of money and support behind people who supoprt them. Gun control leaders like Giffords and Bloomberg are in the process of proving they'll stand behind Republicans who come down on the other side. This month, they took the first step.