WASHINGTON The November 2 midterm elections are likely to put a lot more Republicans in the now Democratic-led Congress.
But it's uncertain if Republicans will win control of the House of Representatives or Senate -- or if President Barack Obama's party will stun the nation and hang on to both chambers.
Democrats currently hold the House, 255 to 178, with two vacancies, and the Senate, 59-41. All 435 House seats and 37 seats in the Senate will be up for grabs on election day.
Here's a look at possible congressional compositions.
* REPUBLICANS TAKE HOUSE, DEMOCRATS KEEP SENATE
Likely. Perhaps the most probable outcome is a divided Congress -- with Republicans taking 218 seats or more in the House to win a majority and Democrats retaining the Senate, albeit with a reduced majority.
That would mean House Republicans could pass legislation on a simple majority vote without any Democratic support.
If that is the case, Republicans may move to quickly approve much of their conservative "Pledge to America" agenda, which includes measures to repeal Obama's overhaul of the U.S. healthcare system, end government control of mortgage giants Fannie Mae and Freddie Mac and impose a freeze on federal hiring.
But if the Democrats' majority in the Senate is big enough, they would likely block most House-passed measures. That would create gridlock, or force the two parties and two chambers in Congress to work together.
If gridlock sets in, perhaps the only legislation that would win approval would be spending bills needed to prevent federal shutdowns and measures that enjoy rare bipartisan support.
One of those bills could be an effort to extend tax cuts implemented during the administration of President George W. Bush that are set to expire at the end of this year.
Obama and most Democrats want to renew tax cuts only for individual annual incomes below $200,000 and for family incomes below $250,000. Republicans want to extend all of the tax cuts, including for the portion of any income above those levels.
Attempts to reduce the budget deficit would also suffer from political gridlock.
* REPUBLICANS WIN HOUSE AND SENATE
Possible but unlikely. Republicans would have to pick up 10 seats -- including a number of them in highly competitive races -- to capture the Senate, 51-49.
That would give them control of both chambers for the first time since 2006, midway through Bush's second term.
But it would still leave Republicans far short of the 60 votes needed to clear Democratic procedural hurdles known as filibusters, not to mention the 67 to override an Obama veto.
Obama could prevent a Republican Senate from repealing major legislative successes, including his healthcare overhaul, crackdown on Wall Street and revamping of the federal student loan program.
But a Republican Senate, working with a Republican House, could try to trim certain provisions, such as funding for a new Wall Street consumer watchdog agency. On such narrow issues, they might get some traction.
Regardless, a Republican Senate would make it tougher for Obama on a key front -- winning confirmation of his nominees, including those for federal courts and various U.S. agencies.
An increased Republican presence would also make it harder -- if not impossible -- for Obama to win climate change legislation to impose mandatory reductions of greenhouse gas emissions from smokestacks and tailpipes.
SENATE SPLITS, 50-50
Possible. The election could split the Senate, with 50 senators in the Democratic caucus and 50 in the Republican caucus.
Democrats would retain control, however, because U.S. Vice President Joe Biden, as Senate president, breaks all ties.
REPUBLICANS WIN HOUSE, AND OVER 60 SEATS IN SENATE
Very unlikely. Sixty seats in the Senate is a key number because a party can avoid filibusters from the rival party if it has 60 Senators.
If Republicans hold all of their 18 Senate seats up for grabs, and win all of the 19 Democratic Senate seats up for election, they will end up with a 60-40 majority next year.
That would be enough, provided all Republicans stick together, to clear Democratic filibusters and give the Republicans a much freer reign to pass bills.
REPUBLICANS WIN HOUSE, AND VETO-PROOF SENATE MAJORITY
Impossible. A party needs 67 senators, or a two-thirds majority, to make laws without fear that the president can veto them. But even if Republicans win all 37 Senate races this year, they will end up far short, at just 60. The only way they could hit 67 on any specific measure would be to get a number of Democrats to cross the political aisle.
DEMOCRATS WIN HOUSE AND SENATE
Unlikely. Democrats insist, at least publicly, that they will retain control of both chambers.
If they pull off such a shock, they would still likely end up with smaller majorities than they have right now.
That means it will be tougher to pass any of Obama's stalled efforts, such as ones to overhaul U.S. immigration laws and curb climate change.
With Americans angry about the economy, Democrats would be certain to focus on trying to reduce what has been a stubbornly high U.S. jobless rate of 9.6 percent.
Representative Chris Van Hollen, a member of the Democratic leadership, said his party will make Obama's $180 billion proposal to spur the economy and create jobs a top priority.
DEMOCRATS KEEP HOUSE, REPUBLICANS TAKE SENATE
Very unlikely. This would be a surprise ending given that opinion polls give Republicans a lead in the House race but less of an advantage in the Senate.
It's another scenario that could create political gridlock.
Nancy Pelosi, one of Obama's closest legislative allies, would probably stay as speaker of the House and Senate Republican Leader Mitch McConnell would emerge as the undisputed leader of his party on Capitol Hill.