The US midterm elections will likely have a profound effect on policy, from climate change legislation to tackling the budget deficit.
With Republicans regaining control of the House of Representatives and bolstering their strength in the Senate, primarily because of voter concern about the weak US economy.
Here are some questions and answers about how the US Congress would handle key issues:
HOW WOULD REPUBLICAN CONTROL AFFECT THE AGENDA?
The party that controls the House or the Senate holds crucial power, taking the lead in writing bills and deciding which to bring up for a vote and when. A Republican House could pass legislation, such as promised tax relief, on simple majority votes and without any Democratic support. But Senate Democrats could block House-passed bills, including any repeal of Mr Obama's healthcare overhaul.
Even if Republicans win control of the Senate, they would need 60 seats to avoid a Democratic "filibuster," a procedural hurdle used to block measures in the 100-member chamber.
Republicans are not expected to get 60 seats in the Senate - or the 67 needed to override presidential vetoes. That could cause gridlock, with Congress passing only mandatory spending bills and non-controversial measures.
WHAT ARE SOME REPUBLICAN PRIORITIES?
House Republican leaders will push their "Pledge to America," a governing agenda they say would create jobs, cut taxes and shrink government.
While short on specifics, the plan calls for saving $100 billion next year by scaling back spending to 2008 levels (with exceptions for the elderly and US troops), ending government control of mortgage giants Fannie Mae and Freddie Mac and imposing a freeze on federal hiring. Mr Obama is unlikely to sign many or any of the Republican proposals into law. But they set markers in what could be a rough fight in the final two years of the president's term.
WHAT HAPPENS TO TAXES?
Tax cuts brought in by former president George W. Bush run out at the end of 2010, on the watch of the current Congress that ends its term in December.
If politicians fail to extend the cuts due to partisan differences, Republicans are certain to try to renew them in early 2011 once the new Congress starts.
Mr 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.
WOULD REPUBLICANS REPEAL OBAMA'S HEALTHCARE LAW?
Republicans very likely will not have the votes to override an Obama veto of any legislation to repeal his healthcare law.
But they could try to cut off funding to prevent full implementation of the landmark program that passed Congress without Republican support.
Republican John Boehner, in line to become House speaker if his party wins control of that chamber, recently said he was committed to do everything he can to overturn the reform.
But Ethan Siegal of the Washington Exchange, a private firm that tracks Congress for investors, said: "It's going to be very difficult for Republicans to defund, defang, kill or impede healthcare reform legislatively."
Mr Obama would veto any bill to repeal and he could also reject any measure that fails to provide funds to federal agencies to implement the overhaul. Failure to agree on spending bills could result in the shutting down of some agencies but that is unlikely. The bigger threats to full implementation of Obama's healthcare overhaul are public opposition and court challenges.
HOW WOULD CONGRESS TACKLE THE DEFICIT?
A presidential panel is set to make recommendations on December 1 on tackling the budget deficit of $1.3 trillion for the fiscal year to September 30th. With voters alarmed about the US fiscal gap and the pace of federal spending, the deficit debate is certain to get renewed attention next year. Republicans favour spending cuts over tax hikes but some deficit hawks say everything should be on the table. Among possible solutions: cutting retirement benefits, reforming federally funded healthcare programs and reforming the tax code.
WHAT HAPPENS TO OBAMA'S WALL STREET CRACKDOWN?
Republicans want to roll back the landmark Wall Street reforms enacted in July but tinkering around the edges may be all they can muster.
Analysts see little to no chance of a full dismantling of the law meant to prevent a repeat of the 2007-2008 financial crisis that set off the worst US recession in generations.
But Republicans are targeting specific provisions of the reforms, such as funding for the new consumer watchdog. On such narrow issues, they might get some traction.
CLIMATE-CHANGE LEGISLATION
Republicans would be positioned to reverse Democrats' already stalled drive for comprehensive climate control legislation.
A Republican takeover of either chamber, or even large gains by Republicans, will make it harder - if not impossible - for Mr Obama to win legislation imposing mandatory reductions of greenhouse gas emissions from industry and vehicles.
That is especially true if next year's Senate is filled by more sceptics of human-induced global warming. Mr Obama would still have the regulatory power to steer the country away from polluting fuels such as coal and oil but Republicans could counter by trying to deny funds.
WHAT ABOUT INVESTIGATIVE HEARINGS?
If they win the House or Senate, Republicans would take over the chairmanships of powerful committees that can hold investigative hearings into administration actions, from the war in Afghanistan to the cleanup of the BP oil spill.
Republicans promise plenty of hearings, including into Mr Obama's $814 billion economic stimulus plan.
ANY SIGN OF ENERGY LEGISLATION?
Congress might manage to pass a bill this year to reform offshore oil drilling practices in the wake of the BP spill in the Gulf of Mexico. If it does not, the effort likely would be revived next year but again face hurdles.
A Congress with more Republicans could see a renewed push to open drilling in protected areas like Alaska's wilderness.
Republicans will push for more government support for the nuclear power industry. The Obama administration has taken some steps in that direction but has made more progress conditional on achieving a broader climate change bill. Democratic cooperation would be needed for either effort to succeed.
One bill with bipartisan support that could advance: legislation requiring electric utilities to generate 15 per cent of their power from renewable sources such as solar, wind, geothermal and hydroelectric by 2021.