With the Senate majority potentially up for grabs in November, left-leaning groups are raising eyebrows by endorsing Republican senators running for reelection.

According to a Wall Street Journal report, two of the groups — Americans for Responsible Solutions and Independence USA — crossed party lines because of the lawmakers' belief that background checks for gun purchases should be expanded.

Americans for Responsible Solutions was founded in 2013 by former Rep. Gabby Giffords, who was shot by a would-be assassin in 2011, and her husband NASA astronaut Mark Kelly. The pair endorsed Republican Sens. Pat Toomey of Pennsylvania and Mark Kirk of Illinois in an op-ed for CNN this week, citing their calls for stronger background checks three years ago.

Independence USA, a liberal PAC started by former New York City Mayor Michael Bloomberg, announced in early August it is supporting Toomey for the same reason.

And earlier this year, the Human Rights Campaign pledged its support of Kirk because of his favorable views on same-sex marriage, reports the Journal.

"You can't blame some of these groups for trying to show some bipartisanship by supporting Republicans, but I'm not convinced it's ever worked," former Harry Reid aid Jim Manley told the Journal.

"It doesn't change the fundamental dynamic: Republicans are still in control."

Lanae Erickson Hatalsky of the left-leaning think tank Third Way added: "These movements are playing the long game. They know that if a Republican can come back and say, 'Taking on the NRA didn't kill me, it helped me in my race,' more Republicans could start to crack open their doors for a genuine conversation."