Two major liberal outside groups have endorsed Republicans in Senate races, frustrating Democrats who see the majority as within their grasp.

The Human Rights Campaign (HRC), which advocates for LGBT rights, and Americans for Responsible Solutions (ARS), which supports stronger gun control laws, are typically aligned with the Democratic Party.

But they crossed party lines this year to reward vulnerable Republican senators who have fought for their issues. Both groups endorsed Sen. Mark Kirk Mark Steven KirkLiberal veterans group urges Biden to name Duckworth VP On the Trail: Senate GOP hopefuls tie themselves to Trump Biden campaign releases video to explain 'what really happened in Ukraine' MORE (Ill.), and ARS additionally backed Sen. Pat Toomey (Pa.).

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Liberal critics of those endorsements question why the groups would help any Republican senators, given that the GOP majority has been opposed to their policy goals.

“The first votes that Mark Kirk or Pat Toomey would cast if they’re reelected would be to make [Sen.] Mitch McConnell Addison (Mitch) Mitchell McConnellGraham: GOP will confirm Trump's Supreme Court nominee before the election Trump puts Supreme Court fight at center of Ohio rally The Memo: Dems face balancing act on SCOTUS fight MORE [R-Ky.] majority leader of the Senate once again,” said David Nir, the political director for the liberal Daily Kos blog.

“Parties govern, not individuals, so endorsing a Republican senator is the same as endorsing Republican control of the entire Senate — and if groups like these don’t understand that, they’re committing political malpractice.”

Democrats have a favorable Senate map in November and need just four seats to take back the majority if Democratic nominee Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE wins the White House.

But the outlook for Democrats has darkened in recent weeks, with Senate races in Ohio and Florida appearing to move out of reach. Democratic leaders this week warned their colleagues that they would not win back the majority if the elections were held today.

The Senate races in Illinois and Pennsylvania are likely must-win for Democrats.

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Kirk’s seat appears to be the one Republicans are most likely to lose, with Toomey’s not far behind.

But Kirk won praise from pro-LGBT rights groups with his decision to become the first Senate Republican to back the Equality Act, which would prohibit discrimination based on gender identity or sexual orientation.

And Toomey got credit for bucking his party and co-sponsoring the failed bid to require background checks for more gun sales after the 2012 mass shooting at a Connecticut elementary school. While few Republicans joined him, gun control groups said Toomey gave the idea bipartisan credibility.

The HRC jumped first in March by endorsing Kirk. In response to pushback from liberals, group President Chad Griffin wrote in an op-ed, “We simply cannot ask members of Congress to vote with us, and then turn around and try to kick them out of office.”

ARS came to Toomey’s defense last month, despite him touting his A rating from the National Rifle Association on the trail this year. His opponent, Democrat Katie McGinty, is a vocal supporter of stronger gun control.

The group also endorsed Kirk, and both he and his Democratic challenger, Rep. Tammy Duckworth, have made gun control a central issue for their campaigns.

In a statement to The Hill, ARS spokesman Mark Prentice praised McGinty as “a passionate and strong voice for gun violence prevention.” But he stressed that the group needs to support Republicans who go against the grain.

“Our country needs more Republican elected officials to stand with the vocal majority of Americans who support steps that help keep guns out of the wrong hands and prevent gun tragedies,” he said.

“In the wake of the tragedy at Sandy Hook School, Senator Toomey stood up to the gun lobby and stood up to many in his own party. While he has not backed every proposal we have supported, as advocates for gun safety laws and safer communities, we’re grateful for Senator Toomey’s leadership.”

An ARS source added that the long-term path toward enacting tougher gun safety laws has to go through both parties in order to overcome a filibuster, so groups have to be open to backing likeminded Republicans.

Sen. Chris Murphy Christopher (Chris) Scott MurphyDemocratic senator calls for 'more flexible' medical supply chain to counter pandemics The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon GOP chairman to release interim report on Biden probe 'in about a week' MORE (D-Conn.), a vocal advocate for stronger gun control laws, said Thursday that while Democrats should “congratulate and applaud Republicans when they do work for us,” he won’t back Toomey’s reelection, having endorsed McGinty.

“So long as Republicans are in charge of the United States Senate, good luck getting these votes on the floor of the Senate in the absence of extraordinary measures like a filibuster,” he said during an event at the National Press Club in Washington.

One Democratic strategist working in a competitive Senate race was sympathetic to the dilemma of advocacy groups seeking to change GOP positions.

“This is a uniquely challenging problem for groups that have aligned almost 100 percent with Democrats but are very eager to expand the tent and encourage moderate Republicans to join them,” the strategist said.

“They face a real test among their donors and members because their mission, in many ways, relies on broadening the number of people they can bring with them. … Both of these groups have seen a huge help from Republican allies in terms of making progress and getting to the table.”

The HRC and ARS, however, have not backed up their GOP endorsements with any ad buys or spending in general election races. But ARS’s general message is being amplified by spending from former New York Mayor Michael Bloomberg’s super-PAC, Independence USA.

Bloomberg’s group is running ads in Pennsylvania connected to Toomey’s work on gun control, including an emotional spot that features a woman whose mother was killed in the Connecticut elementary school attack.

The Democratic strategist acknowledged that bipartisan endorsements are “particularly helpful” in an election where vulnerable Republicans are looking to separate themselves from the top of the ticket. But he warned that Democrats are a safer bet to remain loyal, especially if the political winds shift in the coming years.

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“On the six-year Senate reelection timeline, assuming Toomey wins, he will not have a more Democratic presidential electorate he’s facing; he’ll have an off-year electorate that will be more conservative,” he said.

“He’ll likely face less pressure to move to the left on gun safety issues, so there’s a reasonable concern that they will lose some of the support they’ve had from some of these vulnerable senators when reelection is not with a high-turnout, Democratic-tilting electorate.”

— Lisa Hagen contributed.