Over the past 25 years, Republicans have methodically consolidated power in state legislatures, taking both chambers in every Southern state, flipping long-Democratic Midwestern strongholds and claiming new territory like West Virginia. Heading into the midterm elections, they controlled two-thirds of all state legislative bodies.

Newly energized activists and donors on the left had hoped to begin rolling back that trend this year, and on Tuesday Democrats took a big step, netting about 250 state legislative seats. But their major victories all came in states Hillary Clinton won in 2016. Their road back to simple parity remains long:

Who controls state legislatures

State legislatures controlled by Republicans Democrats or were split.

States highlighted in bold changed status from the previous two years, including in off-year and special elections.

Nebraska is not included because it has a nonpartisan legislature. Minnesota’s legislature was nonpartisan until 1972. States in bold changed status in the previous two years, in some states including off-year and special elections National Conference of State Legislatures, Ballotpedia

Democrats took outright control of seven chambers in six states, leaving Minnesota as the only state with a divided legislature. Those wins are modest compared with 2010, when Republicans captured two dozen chambers ahead of the once-a-decade redistricting process that state legislatures largely control.

“Part of the reason Democrats did not do better on Tuesday was because Republicans mostly drew the lines of the districts they’re still running in,” said Tim Storey, the director of state services for the National Conference of State Legislatures. “That has haunted Democrats the entire decade, getting wiped out in 2010.”

Redistricting is around the corner again, and that’s partly why Democrats have made a bigger push this year. They’re reacting, too, to much of what Republican majorities produced: stand-your-ground gun laws, voter ID requirements, bills limiting the power of unions, and social policies like governing who can use public bathrooms.

“Over the last decade, because there’s no policymaking in Washington, the state of policymaking in America has been set at the state level, and it’s been set by Republicans,” said Drew Morrison, the co-founder of EveryDistrict, a group helping Democratic candidates. “And it’s been set with a pretty aggressive conservative vision of what the world should be.”

Mr. Morrison called Democrats’ gains on Tuesday a “generally impressive haul.” But he said they came on ground the party shouldn’t have ceded in the first place. And Republicans have shrugged off the losses, pointing out that they still hold legislative majorities in all six battleground states Donald J. Trump flipped in 2016.

“The battlefield is very deep into their turf,” said Matt Walter, the president of the Republican State Leadership Committee. “They’re still trying to chip away at the thousand seats they lost over the course of the decade, to crawl out from those historic lows.”

Democrats’ more impressive gains this week came in governor’s races, which will help them blunt the effect of some legislatures still in Republican control:

Which party holds the governor's office

Governorships that were held by a Republican , Democrat or a third party candidate . States highlighted in bold changed status from the previous two years, including in off-year and special elections.

*Races are still undecided. National Conference of State Legislatures, Ballotpedia

Democrats won seven governor’s offices (with races in Florida and Georgia still undecided). They now claim the governor’s mansion in the swing states of Michigan and Wisconsin heading into the 2020 election. And they’ll take unified control of the legislature and executive branch in six new states.

Illinois, with its Democratic-controlled legislature, will now have a Democratic governor. In New York, where Gov. Andrew Cuomo remains in power, the Democratic Party will now control the state Senate for the first time in a decade. (Democratic candidates won a majority of Senate seats there in 2012, but a group of them formed a coalition with Republicans, giving them control of the chamber, and our charts here reflect that Republican control).

As a testament to their dominance over the past two decades, Republicans entering this election held unified power in 25 states — a remarkable shift from when they controlled no single state in 1976. (Last year, they picked up West Virginia when its Democratic governor, Jim Justice, switched parties). Democrats held complete power in just eight states entering this election, after off-cycle elections in New Jersey and Washington in 2017.

Where parties have unified state control

State legislatures and governorships controlled by Republicans Democrats or were split.

States highlighted in bold changed status from the previous two years, including in off-year and special elections.

Nebraska is not included because it has a nonpartisan legislature. Minnesota’s legislature was nonpartisan until 1972. States in bold changed status in the previous two years, in some states including off-year and special elections. *Governor’s races are still undecided. National Conference of State Legislatures, Ballotpedia

Political scientists say that Republicans have become so dominant at the state level because of their focus on organization and alliances with well-funded pro-business groups, like the American Legislative Exchange Council (ALEC). And they’ve risen to power in the states during a time, since the 1980s, when responsibilities have increasingly shifted to the states to set rules for federal programs. Most state legislatures have no filibuster, making action easier there, too.

“By the time that progressives wake up to the fact that conservative networks have been so successful, they are now starting from behind,” said Alexander Hertel-Fernandez, a political scientist at Columbia University who has written a forthcoming book, “State Capture,” covering this period.

Republican state legislators have had more power than Republicans in Congress to derail parts of the Affordable Care Act, by refusing its offer to fund most of a Medicaid expansion. Across a range of other conservative priorities, model bills advocated by groups like ALEC have been adopted in states across the country.

As Republicans have become more dominant in states, the legislation they’ve advanced has also become more similar across state lines, a trend Mr. Hertel-Fernandez and Konstantin Kashin, a data scientist, have tracked with plagiarism detection software. They have found a growing share of bills introduced and enacted with text copied nearly verbatim from outside groups, particularly ALEC but also organizations on the left.

“What we’ve seen effectively is the nationalization of state politics,” said Gordon Lafer, a political economist at the University of Oregon. “All those old clichés that ‘states are laboratories of democracy,’ that ‘all politics is local’ — it has been the opposite in the last decade.”

Scholars suggest that Democrats and their donors might have been slow to respond to these trends in state politics because they’re philosophically more inclined to focus on what the federal government can do — and to be suspicious of “state control.”

“That stems from how Democrats view how government should work,” said Mr. Morrison, the Democratic activist. “Government should work for the broadest group of people. And the best way to ensure that policy works for as many people as possible is to have it happen at the federal level.”