With the US emerging from the worst economic downturn for a generation, the election will be in part a referendum on the Democrats' economic policies. But there are also a range of other issues from healthcare to climate change that, polls indicate, will influence which party voters choose on 2 November.
Unemployment rates of nearly 10% (9.2% in October) are politically unsustainable in the US, and the Democratic Party is expected to pay heavily. It's unfortunate for them that some battleground states, such as Michigan, Nevada, California and Florida, are among the hardest hit.
Republicans argue that the stimulus and jobs packages passed by Congress have largely failed to create new jobs. Democrats say that without the stimulus - and other measures such as the auto-bailout - many more jobs would have been lost. While some Democratic candidates are distancing themselves from the stimulus programme others draw attention to projects in their district or state.
The vast size of the US deficit - $1.4tn in 2009, or nearly 10% of GDP - is a major concern for voters. Republicans argue that Washington is living beyond its means and jeopardising the prosperity of future generations. Many are still incensed over the $787bn price tag of the stimulus bill. Democrats say they did what was necessary to combat a recession handed to them by Republican President George W Bush. But voters are increasingly immune to references to Mr Bush. The deficit is now squarely a Democrat problem.
The Obama administration has proposed reducing the deficit to 3% of GDP by 2015, but has not yet produced a roadmap detailing how to get there. A group of Republican leaders in the House of Representatives released a policy document, entitled the Pledge to America, which argues for cuts in both government spending and taxation. Critics retort that this appears to conflict with their promises not to cut the defence budget or entitlements like Medicare and Social Security.
Cutting taxes has long been a rallying cry for conservatives, the most vocal proponents these days being Tea Party supporters. Bush-era tax cuts are due to expire this year, but Congress has delayed a vote on whether to extend them until after the election.
Republicans want to extend all the tax cuts, which pleases conservatives and the business community - many conservatives would like to see taxes cut even further. Democrats say they will vote to extend the Bush tax cuts on those earning up to $250,000 but allow the tax cuts on wealthier Americans to expire. That promise enrages conservatives, but also disappoints many on the left who would prefer to see almost all the tax cuts lapse.
Much of the healthcare reform package signed into law by President Obama earlier this year has yet to take effect, but it's still an emotive issue. Many Republicans regard it as another example of the Democrats' "big government" philosophy. A growing number, including the Tea Party movement, are pushing for full repeal of the legislation. Elderly people are among the most negative towards the bill, and they are also the most active voters in mid-term elections.
But Democrats are willing to bet that most voters oppose rolling back the ban on insurance companies dropping or denying coverage. From September, insurance companies have not been able to force children off their parents' policies until the age of 26 - one tangible benefit, for some families, that came just in time for voting day.
Since the passage of Arizona's new law cracking down on illegal immigrants, the issue has rarely left the headlines. Many conservatives support Arizona's tough line, and it has the vociferous endorsement of most Tea Party activists. Polls show the legislation has broad support nationwide.
Mr Obama has taken a different approach, stepping up border patrols, but encouraging Congress to pass legislation that will put many undocumented workers on a path to legal status. His administration has mounted a legal challenge to Arizona's law. Democrats hope the dispute will motivate Hispanics to turn out and vote and galvanise support from the left, particularly the bloggers and activists of the "netroots" community. Both constituencies were critical to Mr Obama's 2008 victory.
President Obama is facing criticism from both sides. Those to his left disagree with last year's decision to send more troops to Afghanistan and increasingly question the point of US involvement in the country. On the right, senior Republicans, including former rival John McCain, have castigated the president for setting a July 2011 date to begin withdrawing troops. They believe the deadline sends the wrong signals, encouraging insurgents to wait for Americans to leave and sowing doubts among Afghan civilians about America's commitment to their country.
In November, Republicans will seek to make the issue a question of leadership. Democrats will have a tougher time - they have to convince their anti-war flank to stick by the president.
An investigation into allegedly fradulent foreclosures, which was launched in October by attorneys general representing all 50 states, brought America's dire housing situation back into view. The issue stirs up fear of families losing homes and anger at the government for not foreseeing the housing crisis or doing enough to soften its impact.
The issue of banks possibly acting fradulently cuts both ways. Banks remain a public enemy in the US, and Republicans are mostly seen as not having acted to rein in their power. But Democrats are increasingly blamed for the banks' skittish attitudes when it comes to lending, which affects small businesses and individuals alike.
Politicians do not divide along clear party lines, when it comes to climate change. One major factor is which part of the country they represent. Politicians from rustbelt states - former manufacturing powerhouses such as Ohio and Pennsylvania - are wary of regulation, as are those from coal-mining areas. Politicians elsewhere have to pay take account of voters with a greener outlook, and a growing number of Christian groups concerned about the warming planet.
Both parties agree on the need to improve US energy security, though Republicans are more likely to advocate tapping America's oil, gas and coal deposits than seeking alternative energy sources. Many Republicans are also opposed to pricing carbon emissions, arguing that this would undermine American competitiveness.
Terrorism (National Security)
National security isn't the weakness for Democrats that it once was. President Obama has a few wins under his belt. His administration successfully foiled the Times Square bomb plot and has apprehended numerous terror suspects.
But Mr Obama's aspiration to close the Guantanamo Bay detention facility and try the inmates in American courts still gives rise to arguments. Republicans accuse their opponents of treating dangerous terrorists like common criminals and compromising the safety of Americans at home. Democrats counter that Bush-era policies - such as indefinite detention and so-called enhanced interrogation techniques - were un-American. On this one, polls indicate that the public is leaning toward the Republicans.