WASHINGTON - Voters hoping to decide control of Congress might be watching the wrong elections.

While the focus is on November's races, critical House contests in the Philadelphia region may have been most influenced by low-profile state elections in 2010.

That's when Republicans stormed to control of state legislatures, just in time to redraw congressional maps and tilt key districts in their favor for a decade, in Pennsylvania and across the country.

The long-term future of those districts, and of the House, may also depend on state-level races that will decide who makes the next maps, in 2020.

"In a word, it's vital," said Charlie Gerow, a Republican consultant based in Harrisburg.

Lawmakers' power to craft districts, he said, "impacts the political process more than anything else the legislature does."

Consider that three Philadelphia-area suburban districts that as recently as 2010 were held by Democrats (remember Joe Sestak?) now appear firmly in Republicans' hands after redistricting. National Democrats canceled ad buys in a Bucks County-based race and did not seriously contest one in Delaware County, and in a moderate South Jersey district, their nominee trailed badly in a recent poll.

In another once-competitive district centered on Chester County, national Democrats have all but conceded.

The trends this year stem in large part from a national atmosphere favoring the GOP. But even when Democrats had the edge in 2012, Republicans still easily kept the four seats, helped by a tilted playing field.

"It's the beginning and the middle and the end of any conversation about any district anywhere," said T.J. Rooney, a former Pennsylvania Democratic chairman.

That's why Democrats are gearing up for the next wave of state races even as experts debate whether district lines alone alter control of the House.

The Democratic Legislative Campaign Committee launched "Advantage 2020" this year, aiming to win legislatures with the explicit goal of running the next round of redistricting.

"In terms of long-term strategic planning, we're going to need to put more focus and emphasis and investment on 2020 redistricting," said U.S. Rep. Steve Israel (D., N.Y.), who heads House Democrats' national campaign efforts.

The GOP was ahead of the game in 2010, when the Republican State Leadership Committee pushed its "REDMAP" initiative.

Helped by a national wave, 20 of the nation's 99 state legislative chambers flipped to Republican control just before decennial redistricting, said Matthew Walter, the committee's president.

In many states, including Pennsylvania, the maps are approved by state legislators, empowering any party that controls both chambers.

In Pennsylvania, the result was what a columnist on the website Real Clear Politics called "the gerrymander of the decade."

(Even some Democrats grudgingly admired the GOP's handiwork, saying privately they would have done the same given the chance.)

In New Jersey, mapmaking is evenly split between the parties, but Republicans still won the 2011 redistricting fight, pushing the competitive South Jersey seat farther right.

The impact of such shifts can be profound.

When Democrats held three Philadelphia-area swing seats in 2010, they had the majority needed to pass the Affordable Care Act.

Republicans have stymied the president since winning back those seats and the House.

Those victories came before redistricting, a reminder that candidate quality and issues matter, too.

But with redistricting, the GOP added protection.

Democrats won 51 percent of the congressional vote between the parties in 2012, but just 46 percent of House seats, according to David Wasserman of the nonpartisan Cook Political Report.

In Pennsylvania Democrats won 51 percent of the major-party vote, but just five of 18 House seats.

Exactly how much of that outcome was due to partisan manipulation, though, is a subject of intense debate.

Analyst Sam Wang says his data show that Republicans won 10 to 15 more seats nationally than they would have with nonpartisan boundaries.

To Wang, founder of the Princeton Election Consortium, the maps had as much influence as years of natural demographic shifts that have packed liberals into cities.

That "the stroke of a pen" could match nationwide population migration, Wang said, "is a testament to the power of redistricting software."

Others, such as the University of Michigan's Jowei Chen, downplay gerrymandering's

When factoring in maps that skew Democratic in places such as Illinois and Maryland, Chen said, the net effect of redistricting gained Republicans five to eight seats they might otherwise have lost - not nearly enough to undo the GOP's 33-seat advantage.

The main "bias" against Democrats, Chen said, is geography - many of their voters now live in dense cities such as Philadelphia, leading to big victories in relatively few races.

"Democratic voters are very inefficiently clustered," Chen said. "Republicans are spread out, geographically, in a more efficient manner."

While he and Wang differ on the national picture, they agree on this: Maps can make a major difference in a given state or race.

Both point to Pennsylvania as a glaring example.

Philadelphia voters are hemmed into three deep-blue districts, but districts in two neighboring counties lunge and lurch into exurban and rural areas, tinting them red even as Democratic registration has surpassed the GOP in the suburbs.

The notorious Seventh District, which includes much of Delaware County, looks like a couple dancing or a plane refueling, Wang said.

The result: Three GOP seats in the Pennsylvania suburbs effectively offset three Democratic seats in Philadelphia even though Democrats in 2012 outpolled Republicans in those six districts by 430,000 votes.

Compare that with Illinois, where Democrats drew the boundaries. They linked pieces of Chicago with its suburbs, producing several safe, moderate Democratic seats, Chen said - the opposite of Pennsylvania.

Using a program that drew neat, compact districts without regard to party or racial criteria, and applying historic voting patterns, Chen estimated that Pennsylvania's map gives the GOP a half-seat advantage it would not have in a neutral setup.

While seemingly small, that means some districts that might lean Democratic or be toss-ups instead favor Republicans.

"It creates safer seats when we otherwise would have had closer elections," Chen said.

Primaries in strongly liberal or conservative districts push lawmakers far right and left.

"It perhaps impacts the ideological makeup of who gets elected, regardless of party," said Michael Li, redistricting counsel at New York University's Brennan Center for Justice.

Arizona, California, Idaho, and Washington have moved to independent redistricting panels, Li said. Florida voters passed an initiative imposing criteria meant to limit gerrymandering.

In most cases, though, state lawmakers are reluctant to give up power, so reforms are achieved only through voter-driven ballot initiatives, Li said.

For now, those hoping to help their party win the House might do well to look down the ballot.

"That's where the game begins," Chen said. "If you lose that part of the game, then you're just fighting an uphill battle for the rest of the decade."

To the victors go the mapping software.