For decades, Republicans have outmaneuvered and outspent Democrats in state legislatures, gerrymandering them into the minority in both red states and political battlegrounds.

G.O.P. state lawmakers have used that advantage to pass countless conservative policies — with a lot of help along the way.

In back rooms and behind the scenes, conservative think tanks and other policy groups like the Heritage Foundation and the American Legislative Exchange Council drafted model legislation for Republican lawmakers to cut taxes, expand gun rights and loosen environmental regulations.

Now Democrats are trying to put themselves on even footing.

An increasingly prominent player in this liberal push is a little-known group called the States Project, which was founded in 2017 and made a financial splash in state legislative elections last year, pouring $60 million into races in five competitive states: Arizona, Michigan, Maine, Nevada and Pennsylvania.

That investment reflects a broader recognition by Democrats that with Congress often deadlocked, many of the nation’s most urgent battles over abortion access, gun control and voting rights are now unfolding in state capitols. Liberal groups, including issue-based ones like Everytown for Gun Safety and Planned Parenthood, now direct more of their attention to state legislative races.

The States Project, however, focuses solely on them.

“They are very unique and filling a vacuum,” said Joanna E. McClinton, the Democratic speaker of the Pennsylvania House of Representatives, where her party holds a one-seat majority.

Ms. McClinton recalled inviting the States Project to hold a seminar for her caucus in 2021, when her party was still out of power in the chamber and she had just become the minority leader. The group’s presentation included a sweeping list of liberal policy goals, including investments in public education, paid family leave and measures to combat climate change and institute equal pay.

When Democrats unexpectedly took narrow control of the House in last year’s elections, she said, they had a running start. Even with the Senate still in Republican hands, Democrats managed to pass a state budget with a $567 million increase in school funding, including free breakfast for all public school students.

That liberal success, Ms. McClinton, said, could be attributed in part to the States Project.

“The work they did with us while we were in the minority was very crucial in helping us to have a governing one-seat majority in a split legislature,” she said.

Taking full advantage of even a razor-thin majority has become crucial in a country where 39 states are now fully controlled by one party — the most in at least three decades. That one-sidedness means that in many places lawmakers are pushing unflinchingly partisan agendas.

Last year, Democrats flipped one or more chambers in several states, including Michigan and Minnesota, and they have acted quickly, passing measures to tighten gun laws, set limits on carbon emissions, increase education funding and protect abortion access.

The States Project has had a central role. The group, founded six years ago by Adam Pritzker, a businessman and major Democratic donor, and Daniel Squadron, a former New York legislator, has sought to focus its ample resources and attention exclusively on state legislators, trying to fill the void on the left.

“Going back to 1972, the right had seen the extent to which state legislatures were a place that they could impose their worldview,” Mr. Squadron said, noting that Heritage and ALEC were both founded the next year. “The fact that there’s no glamour, and you’re not going to get a presidential candidate sitting on your living room couch by doing this work, the fact that you’re not going to be the top rung of the Beltway, didn’t matter to them structurally, because the return was just too good.”

Mr. Pritzker, who is a cousin of Gov. J.B. Pritzker of Illinois, said, “If you take yourself back to 2017, ’18, ’19, when we got started, it just seemed like this niche backwater, at least in our part of the world, that really no one cared about.”

The inspiration for the States Project began in Albany, where Mr. Squadron, a newly elected lawmaker, was part of the class of Democrats that first took over both chambers in 2008.

It was not smooth, as he recalls.

“It was a mess,” he said. “We were on the front page of The New York Post with clown faces superimposed over our own faces.”

In the next cycle, Republicans retook control. “Worst of all, we lost the majority,” Mr. Squadron added, “and didn’t just lose the majority, but lost the majority in a way where the Democratic governor used us as an example of what not to be and didn’t recover from that for years and years.”

Last year, the States Project made large donations to Democratic state legislators’ campaigns across the country. The group’s founders saw it as a shrewd investment.

“When we were initially building our budget, the total cost to run a slate of candidates in Virginia was something like in the hundreds of thousands of dollars,” Mr. Pritzker said, adding that the last presidential election cost about $14 billion. “The delta there is astounding. How’s that even possible where there’s an asymmetry like that? In terms of the power of the states, the power of having a majority, of passing laws that improve people’s lives in the state.”

Mr. Squadron and Mr. Pritzker are quick to point out differences between the States Project and its conservative counterparts. For one, their group doesn’t lobby lawmakers to pass specific bills or have lobbyists on the payroll. And its efforts in state capitols have a much smaller budget: roughly $5 million per year, with about 20 full-time staff members working in 15 states.

The group is funded by Future Now Action, a tax-exempt, so-called dark-money group that is not required to disclose its donors. The States Project declined to reveal its donor list.

While most organizations seeking to craft state policy share their model legislation and plans out of the public eye, the States Project posts all of its proposals on its website.

“When people get elected, there’s no real road map to being able to take the things that you campaigned on and turn them into real, tangible efforts and good governance and policymaking,” said Erika Geiss, a Democratic state senator in Michigan. She said the States Project’s online policy library had been helpful for drafting the final version of a bill she wrote on paid family leave. (The bill hasn’t advanced, but Gov. Gretchen Whitmer of Michigan, a Democrat, mentioned it as a top priority in her “What’s Next” address last month.)

In some cases, lawmakers have taken cues from their peers in other states.

In Pennsylvania, as Democrats sought their expansion of the school meal program, State Representative Matthew Bradford, the party’s majority leader, said that he looked to similar efforts in Michigan for a blueprint.

“Obviously they do have a trifecta, but I believe they also have the distinction of being a Trump 2016 state,” Mr. Bradford said. Michigan, he added, showed how “you can do the possible” and offered a road map for “synthesizing the general ideas.”

One popular States Project program: a message-board-meets-liberal-dating-service interface that allows the group’s officials to identify state lawmakers in different places who are most likely to help one another on a project.

“A quick call to someone in the States Project saves me time,” said Sarah Anthony, a Democratic state senator from Michigan who serves as chair of the budget committee. Otherwise, she added, “My team and I try to scour the internet to try to figure out who’s tackling which problems where.”

Lawmakers can also provide accounts of their experiences on the portal.

“I was the chief author of our 100-percent-clean-energy-by-2040 bill in Minnesota,” said Jamie Long, the Democratic majority leader in the Minnesota House. “And so the States Project had me on a panel conversation with legislators from other states to talk about how we did it, what some of our secrets to success were and used us as an example.”

With Democrats likely to continue to focus on abortion rights as a key issue in 2024, the States Project is girding for another busy year in state legislatures.

“Abortion was definitely an important catalyst that brought an enormous amount of attention to the states,” Mr. Pritzker said. “I don’t think any of us expected that there would be a spotlight on states the way there is now.”