By John Whitesides

WASHINGTON (Reuters) - A Republican win in even onechamber of the U.S. Congress would create a huge shake-up inWashington, with far-reaching implications. Here are somequestions and answers about the Nov. 2 midterm election and theimpact of a potential Republican victory.

WHAT IS THE MOST LIKELY RESULT?

With two months to go, political handicappers are pickingRepublicans to narrowly win a House majority and make big gainsin the Senate, although not enough to give them control.

Charlie Cook of the nonpartisan Cook Political Report wroteon Tuesday that his race-by-race analysis showed Republicansare poised to gain at least 40 House seats and possibly more.

"The House has reached the tipping point," Cook wrote.

University of Virginia political scientist Larry Sabatoraised his prediction for Republican gains last week to 47House seats.

In the Senate, Cook predicts a Republican gain of seven tonine seats. Republicans must retain their own Senate seats andsweep nearly all of the competitive Democratic seats to wincontrol, a difficult.

WHY ARE DEMOCRATS IN SUCH TROUBLE?

Worries about the economy, unhappiness with Democraticleadership in Washington and plummeting confidence in PresidentBarack Obama have created a perfect political storm.

Obama has seen his approval ratings slide well below 50percent over the summer, while the number of Americans whobelieve the country is on the wrong track has climbed to above60 percent in most polls.

Most polls show the sweeping healthcare overhaul passed byDemocrats in Congress remains unpopular with a majority ofAmericans, and public dissatisfaction with congressionalDemocrats is widespread.

Obama's popularity among independents, a crucial bloc thatbacked him in 2008, has dipped below 50 percent in many polls,and Republicans are more enthusiastic about the election andmore likely to vote than Democrats.

Obama and Democrats got no help last week from the latestjobless report, which showed the unemployment rate inching upto 9.6 percent.

WHAT KEY RACES COULD DECIDE HOUSE AND SENATE CONTROL?

In the House, Democrats elected in Republican-leaningdistricts in 2008 with the help of a heavy Democratic turnoutfueled by Obama's candidacy are struggling to hold their seatsin the face of a growing Republican wave.

The fate of first-term Democratic incumbents likeVirginia's Tom Perriello, Colorado's Betsy Markey, Ohio's JohnBoccieri and Florida's Alan Grayson could be crucial to thebalance of power.

In the Senate, a handful of veteran Democrats facingunexpectedly tough re-election races could decide the outcome,including Democratic leader Harry Reid of Nevada, Russ Feingoldof Wisconsin, Patty Murray of Washington and Barbara Boxer ofCalifornia.

Democrats are keeping an eye on two races they thoughtwould be cakewalks, the Connecticut battle for retiringDemocratic Senator Chris Dodd's seat and the election toreplace late West Virginia Senator Robert Byrd.

If Republicans pull a surprise win in either state it couldbe the end for Democratic Senate hopes.

WHAT WOULD REPUBLICAN LEADERSHIP IN CONGRESS MEAN?

A Republican win in either the House or Senate, or both,could mean gridlock and political conflict -- and plenty ofit.

Republican majorities would slam the brakes on Obama'slegislative agenda, but would find it nearly impossible to passnew initiatives that did not have considerable Democraticsupport.

Even if Republicans muscled partisan bills through theHouse, Democrats in the Senate would be able to block themthrough either majority votes or a procedural tactic that takes60 votes to overcome.

That could mean Republicans will be forced to work withmoderate Democrats in both chambers if they want to passlegislation -- particularly since Obama will be waiting to vetoany bill that does not pass muster among Democrats.

Republican majorities would put House leader John Boehnerand Senate leader Mitch McConnell in charge, and they would setthe agendas for their chambers while trying to manage moreconservative Republican caucuses.

Republicans also would run committees and gain subpoenapower that would make it easy to investigate the Obamaadministration and force witnesses to testify.

WHAT ISSUES WOULD A REPUBLICAN CONGRESS FOCUS ON?

On the campaign trail, Republicans have pushed lowerspending, the extension of expiring tax breaks and the repealof the healthcare overhaul.

Undoing key elements of the Obama agenda -- the stimulusspending and the healthcare bill -- would be a high Republicanpriority. Democrats, however, probably would have enoughbacking to block the moves in the Senate, and Obama would vetothe effort anyway.

On foreign policy, Obama's troop escalation in Afghanistandrew support from most Republicans, with the prime oppositioncoming from his fellow Democrats. His planned troop withdrawalnext year would likely draw Republican opposition, however.

WHAT DOES IT MEAN FOR THE 2012 WHITE HOUSE RACE?

A Democratic loss would be a bruising blow to Obama, whoentered the White House with high public hopes in January 2009.He has helped push through Congress a broad economic stimulusbill and ambitious overhauls of the healthcare system andfinancial regulations.

But the loss also would free Obama to engage in directbattle with Republican congressional leaders and presidentialcontenders, giving him a new set of political foils andsomewhere else to point the finger of blame when things gowrong.

Obama would have a role model in former President BillClinton, a Democrat who saw Republicans capture Congress in a1994 sweep two years into his term.

But Clinton easily won re-election in 1996 after moving tothe center and winning a budget showdown with Republican HouseSpeaker Newt Gingrich over the closing of the federalgovernment.

WHAT IS AT STAKE FOR THE REPUBLICAN PARTY?

Win or lose, Republicans face an internal battle over thefuture of the party after this year's success of the grassrootsconservative "Tea Party" movement.

The movement, which favors limited government, low taxesand less government spending, could be well represented in thenew Congress and push the party and its presidential contendersto the right.