Scenarios: Possible outcomes of congressional elections

WASHINGTON (Reuters) - The midterm elections on Tuesday are likely to put a lot more Republicans in the currently Democratic-led Congress.

The opposition is likely to win control of the House of Representatives and pick up seats in the Senate, according to opinion polls.

Democrats now have a 255-178 advantage in the House, with two vacancies, and they control the Senate by a 59-41 margin. All 435 House seats and 37 seats in the Senate will be up for grabs on election day.

Here is a look at possible outcomes of the congressional elections.

* REPUBLICANS WIN 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 control of 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 they win a majority, House Republicans may move to quickly approve much of their conservative "Pledge to America" agenda, which has measures to repeal Obama's overhaul of the healthcare system, end government control of mortgage giants Fannie Mae and Freddie Mac, and impose a federal hiring freeze.

But if the Democrats' majority in the Senate is big enough, they would likely block most House-passed measures, creating gridlock or forcing the two parties to work together in the House and Senate.

If gridlock were to set in, the only legislation winning approval might be spending bills needed to prevent federal shutdowns and measures enjoying rare bipartisan support.

The 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 could be a potential area of compromise.

Obama and most Democrats want to extend tax cuts only for individuals earning below $200,000 a year and for family incomes below $250,000. Republicans want to extend all of the tax cuts, including for any income above the $200,000 and $250,000 levels.

Attempts to reduce the budget deficit could also suffer from political logjam.

* REPUBLICANS WIN HOUSE AND SENATE

Possible but unlikely. Republicans would have to pick up 10 seats -- including a number in highly competitive races -- to have the 51 votes needed to control the Senate.

But they would still fall far short of the 60 votes needed to prevent Democrats from blocking legislation by using procedural hurdles known as filibusters, not to mention the 67 votes needed to override an Obama veto.

Obama could prevent a Republican Senate from repealing major legislative successes, including his healthcare overhaul and financial sector regulation.

But a Republican Senate, working with a Republican House -- the party last controlled both chambers in 2006 -- could try to trim provisions, such as funding of a new Wall Street consumer watchdog agency. On such narrow issues, they might get traction.

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.

And it would be harder -- if not impossible -- for Obama to win approval of climate-change legislation to impose mandatory reductions of greenhouse gas emissions from smokestacks and tailpipes.

SENATE SPLITS EVENLY

Possible. Democrats, however, would retain control because U.S. Vice President Joe Biden, as Senate president, would cast all tie-breaking votes.

REPUBLICANS WIN HOUSE AND AT LEAST 60 SEATS IN SENATE

Very unlikely. Sixty Senate seats is a key number because it would allow the majority party to avoid filibusters, provided all its members are united.

If Republicans hold all of their 18 Senate seats that are contested and win all of the 19 Democratic Senate seats up for grabs, they would end up with a 60-40 majority next year.

REPUBLICANS WIN HOUSE AND A VETO-PROOF SENATE MAJORITY

Impossible. Even if Republicans were to win all 37 Senate races this year, they would come up short. The only way they could get the 67 votes needed to override Obama's vetoes would be to persuade Democrats to cross the political aisle.

DEMOCRATS KEEP CONTROL OF THE HOUSE AND SENATE

Unlikely. Even if they were to pull it off, Democrats would still likely end up with smaller majorities in both chambers, making it tougher to pass any of Obama's stalled legislative efforts, including an overhaul of immigration laws and climate-change regulation.

With Americans angry about the economy, Democrats would be certain to focus on trying to reduce what has been a stubbornly high national jobless rate, currently at 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, as opinion polls give Republicans a lead in the race for the House but less of an advantage in the campaign for the Senate.

This scenario also could create political gridlock. House Speaker Nancy Pelosi, one of Obama's closest legislative allies, would probably keep her job, while Senate Republican Leader Mitch McConnell would become the majority leader and emerge as the undisputed leader of his party in Congress.

(Editing by Leslie Adler)