US mid-term elections: What is at stake?
The US mid-term elections that will define the shape of American politics for the next two years and beyond are around the corner. Here is an introduction to the campaign and a look at what is at stake.
The elections - so-called because they occur in the middle of a presidential term, are typically low-turnout affairs. Without a marquee presidential race on the ballot, the mid-terms attract fewer voters to the polling stations.
But mid-term elections can have a long lasting impact on the US government - this time, Mr Obama's ability to govern in the final two years of his tenure in the White House could be shaped by what happens on 4 November.
Who, if not the president, is on the ballot?
Thirty-six out of 50 state governors, a little more than one third of the 100-seat Senate, all 435 members of the House of Representatives, and countless state and local offices are up for election.
The most keenly watched race is for control of the Senate. The Democrats have the majority now, but history and the prevailing political climate suggest the Republicans stand a good chance of taking over.
Currently, the Democrats hold a five-seat majority (53 seats and two independents who caucus with them, compared to 45 seats for the Republicans).
That means the Republican Party need to win six seats to take control of the Senate. They already control the House of Representatives and are unlikely to lose it.
Why is the climate so favourable to the Republicans?
The Senate seats up for election this year were last in play in 2008, a banner year for the Democratic Party.
With Mr Obama on the ballot that year, energising Democrats who also voted for the party's senate candidates, the Democrats picked up eight Senate seats. Six years later, the party must defend those seats and others, but in a significantly less friendly environment.
Of the 21 Senate races considered competitive, the Democrats are defending 17 seats, compared to four for the Republicans.
There is a longer-term trend to consider - historically, the party that controls the White House loses strength in Congress in mid-term elections.
And finally, in addition to the vagaries of the political calendar, voters weary with the still-sluggish economy, endless conflict in the Middle East, and political polarisation in Washington may take their anger out on the party in power - Mr Obama's Democrats.
What happens if the Republicans win the Senate?
The day-to-day running of the chamber will fall to Mitch McConnell, currently the minority leader.
As Senate majority leader, he will set the chamber's agenda, deciding which bills come to the floor and managing floor debate to advance the party's agenda.
Republicans will also take on committee chairmanships, giving them significant authority to launch investigations and shape the policy debate in the Capitol.
What will that mean for the rest of Mr Obama's term?
Partisan gridlock in Congress has reached historical levels.
In terms of the number of bills passed, the current Congress is the least productive in recent history.
That's because while the Democrats nominally control the Senate, the chamber's rules give the minority party significant power to block legislation, if not to advance their own agenda.
If the Republicans win the Senate, the party could begin to pass a wish-list of legislation but few would be signed into law because Mr Obama would simply veto the bills with which he disagrees.
But a Republican-controlled Senate could have influence in other ways - it could refuse to confirm Mr Obama's appointments to judgeships, ambassadorships, cabinet positions and lower-level administration jobs.
That would make it very hard for Mr Obama's government to operate, and could limit Mr Obama's ability to shape the federal judiciary.
A Republican Senate could also launch investigations of the White House the way the House of Representatives has since 2011.
What about the governors' races?
Governors have significant influence over the politics in their state capitals - and the lives of their constituents - especially at a time when so little is happening in Washington.
For instance, many Republican governors have refused to accept the billions of dollars in federal aid to expand public health insurance rolls under Mr Obama's 2010 healthcare overhaul.
Those governors say they fear their states will be stuck with the bill should Washington one day withdraw the money - but Democrats charge they are denying health insurance to millions of people just to make a political point.
And in the critical presidential swing states such as Florida and Pennsylvania, governors can mobilise the party's network of donors, volunteers and local activists, aiding their party's candidate.
Which recent midterms have made a difference?
In 1994 the Republican Party took control of the House and Senate, setting the stage for six years of battles with Democratic President Bill Clinton.
In 2006, the Democrats retook both chambers, enabling President Barack Obama to accomplish significant parts of his agenda when he won office two years later.
But in 2010, the Republicans took back the House, curtailing Mr Obama's ability to realise his vision for the US.