Call it a ripple-down effect that could determine House control.

The outcomes of marquee races always influence down-ballot contests on Election Day, and, this year, whether Democrats keep power or Republicans seize it could well hinge on which party prevails in several hotly contested races for governor and Senate.

With just over three weeks before Nov. 2, Republicans hope strong-running statewide campaigns in places like Pennsylvania, Nevada, Wisconsin and Ohio will help them triumph in on-the-bubble congressional races _ and help them win the House. Democrats, meanwhile, look to top-of-the-ticket wins in places like Colorado, Maryland and California to save them from total disaster.

Both parties acknowledge that victories at the gubernatorial and Senate level could pull their candidates over the line in some of the 75 or so competitive House races on a playing field that, while still favoring the GOP, is growing even more volatile as Election Day nears.

Coattails also could influence statehouse races nationwide. And the party that controls the legislature controls the redrawing of congressional districts for the next decade. Democrats are in charge of 60 of 98 state House and Senate chambers; the GOP is confident of gains. Legislatures are controlled by a narrow majority in several states, including Nevada, Ohio, Texas and Pennsylvania _ all states that have high-profile statewide races.

From coast to coast, Republicans have the upper hand in races and are likely to gain seats up and down the ballot given a troubling climate for Democrats. The GOP is looking to its strength among gubernatorial nominees, in particular, to help it pick up the 40 seats it needs to win power in the House. It's more likely than the Senate to change hands; the GOP would need to gain 10 seats there.

"In a midterm election where turnout could be a key factor in deciding which party holds a majority in Congress, enthusiasm for GOP candidates at the top of the ticket will benefit Republicans in battleground districts," said Rep. Pete Sessions of Texas, the chairman of the House Republicans' campaign committee.

But David Plouffe, a Democratic National Committee senior adviser, predicted that Democrats will do better than expected in key races. Said Plouffe: "If we continue to show progress gradually ... we're going to win some of these close elections. We're going to surprise people."

Republican and Democratic strategists say House control is up for grabs, with races growing more competitive as voters pay closer attention and Democrats rally behind their candidates. Both parties are shifting strategies daily; Republicans are broadening their footprint to more Democratic-held districts, while Democrats are narrowing their focus in hopes of hanging onto a slim majority.

In a midterm election, gubernatorial and Senate races boost turnout, and many people still vote a straight ticket for a party, not a person. That's true of both die-hard Democrats and Republicans who turn out in heavier numbers than unaffiliated voters in midterms and others who may be well-versed in the big races but know little about other contests.

The GOP has significant opportunities to win governorships across the Great Lakes states, a recession-weary region where Democratic House candidates are in jeopardy. Polls also show Republicans favored to take the biggest prizes in other states with competitive House races.

Among such races for governor are:

_Pennsylvania, where Republican candidate Tom Corbett is running ahead of Democratic rival Dan Onorato. Republican Pat Toomey also has an edge in the Senate fight. Double-barreled Republican victories could help GOP candidates win up to six Democratic-held House seats, including those held by Kathy Dahlkemper, Patrick Murphy and Chris Carney.

_Wisconsin, where Republican Scott Walker is leading Democrat Tom Barrett. Democratic Sen. Russ Feingold also is in a tough re-election battle. House Democrats are looking to keep the retiring Rep. David Obey's seat. Democratic Rep. Steve Kagen's re-election bid also is suffering.

_Ohio, where former GOP Rep. John Kasich is trying to fend off an ascendent Democratic Gov. Ted Strickland. It's all but certain Republican Rob Portman will win the Senate race. No less than five Democrats could lose their House seats, with Reps. Steve Driehaus and Mary Jo Kilroy most endangered.

_Iowa, where GOP former Gov. Terry Branstad is comfortably ahead of Democratic Gov. Chet Culver. Veteran Democratic Rep. Leonard Boswell is facing a threat from Republican Brad Zaun.

_Nevada, where Republican Brian Sandavol is favored to beat Democrat Rory Reid. His father, Senate Majority Leader Harry Reid, is in a tight re-election battle, too. So is freshman Democrat Rep. Dina Titus.

Republicans also point to Oregon and New Mexico as other states where polls show GOP gubernatorial candidates performing well.

Elsewhere, Democratic House candidates are appearing to benefit from their party's strength at the top of the ticket in a handful of states. Among those races for governor are:

_Colorado, where polls suggest Democrat John Hickenlooper is a shoo-in to win over Republican Dan Maes and third-party candidate Tom Tancredo. Democratic Sen. Michael Bennet also has seen his standing improve. The victories could help Rep. John Salazar in his competitive race _ and possibly even save Rep. Betsy Markey's uphill re-election bid.

_Maryland, where Democratic Gov. Martin O'Malley is fending off a challenge from Republican former Gov. Robert Ehrlich. In this state, Democratic Rep. Frank Kratovil is still in a fight that even his own party says should have been over by now.

_California, where Democrat Jerry Brown is locked in a battle with Republican Meg Whitman for governor, and where Democratic Sen. Barbara Boxer has recently seen her re-election prospects strengthen. Reps. Jerry McNerney and Loretta Sanchez would stand to benefit from Democratic triumphs.

Democrats also say the lack of governor's and Senate races in Virginia and New Jersey also may be helping two freshmen congressmen. Reps. John Adler in New Jersey and Tom Perriello in Virginia both are hanging on even though Republicans have assailed them for months.

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Associated Press writer Julie Hirschfeld Davis contributed to this report.