CHICAGO (Reuters) - As Democrats struggle to keep their majorities in the U.S. Congress in the November 2 elections, the next decade of party control may well be determined by scores of state legislative races also on the ballot.

At stake is the redrawing by state lawmakers of electoral districts for the House of Representatives in Washington -- an adjustment of boundaries every 10 years that tends to favor the party in charge of each state legislature.

The Democratic Party, which has gained seats at the state level since 2004, faces a Republican onslaught expected to alter the balance of power in Congress and in the states.

“The theme for the Democrats is what goes up must come down,” said Tim Storey, an elections analyst at the bipartisan National Conference of State Legislatures, noting the party in control of the White House has lost state legislative seats in 25 of 27 midterm elections since 1900.

“The real question is how big will the losses be for the Democrats? Where will they be? How critical will they be?”

State legislatures assert themselves in various ways -- their proposals can percolate up to Congress and they may throw up roadblocks to national policies.

For instance, a handful of state legislatures have sought to block enforcement of the healthcare overhaul championed by President Barack Obama, with Republican opponents arguing it is unconstitutional to require people to buy health insurance.

GERRYMANDERING

In most states, legislatures are in charge of redrawing congressional districts once each decade to take account of population shifts recorded by the U.S. Census. The party in power can draw friendly, and convoluted, boundaries to include voters who support its congressional candidates.

Big Republican wins in state houses could give them control of the gerrymandering for about 165 of the 435 seats in the U.S. House, compared with just 30 or so for the Democrats, a report by the National Council of State Legislatures said.

The NCSL projects that control is likely to switch mainly in favor of Republicans in at least 12 chambers, with states such as Alabama, Colorado, Indiana, New York, Ohio, Pennsylvania and Wisconsin in play.

“If one party controls both houses of the legislature and the governorship then they’re in the catbird seat as far as redistricting is concerned,” said Dennis Goldford, professor of politics at Drake University.

The results of state legislative elections could have “ramifications for the whole decade” as they could reinforce one party’s strength in the U.S. House, he said.

In 2006, the Supreme Court largely upheld a controversial redistricting plan in Texas that led to more Republicans than Democrats being elected to Congress from that state.

Gerrymandering in the 1990s in Illinois ensured a safe Hispanic district for Democratic Representative Luis Gutierrez by connecting geographically distant chunks of Chicago and some suburbs by way of an interstate highway.

On November 2, American voters will elect all 435 members of the House and 37 of the 100 senators. Opinion polls show Republicans look likely to win the House but fall short of taking control of the Senate.

At the same time, voters will pick 37 state governors and fill more than 80 percent of the nation’s 7,382 state legislative seats.

Democrats now control both chambers in 27 state legislatures and Republicans are dominant in 14. Control is split in eight states, while Nebraska’s single-chamber legislature is nonpartisan.

MONEY FLOODS IN

With so much at stake, both parties are pouring millions of dollars into states where they stand to gain or lose seats.

The Democratic Legislative Campaign Committee is looking to raise $20 million, said executive director Michael Sargeant.

“From all the polling I’m looking at, it’s a very competitive political environment and I think that we’re going to hold the majority of our majority,” he said.

The Republican State Leadership Committee, which plans to spend $18 million on state legislative races this year, is predicting the party will gain 10 chambers without losing any now under its control, said Chris Jankowski, executive director of the committee’s redistricting project.

“We’ve been planning for over a year now to raise extra money and target the state legislatures we think will have the biggest impact on congressional redistricting.”

In a few states, voters will also be asked to weigh changes to the redistricting process itself.

There are two conflicting measures in California, which has the largest number of congressional seats of any state. One would remove the state legislature’s power to redraw congressional districts and instead have a commission now responsible for state legislative districts do it.

“That would be a major change,” Storey said.

The rival proposition would eliminate the commission altogether and keep the responsibility with the legislature.

Florida voters will be asked to set standards for the process, including a prohibition against redrawing boundaries designed to help or hurt an incumbent or political party, while requiring districts to be contiguous and compact.