There are few better ways to understand the issues and mood of an American election than watching its ads. This year is no exception.

Some commentators have dubbed 2010 the nastiest campaign ever. But several attack ads - like Jack Conway's infamous "Aqua Buddha" spot which questions his rival's faith - have backfired, leaving the attacker trailing in the polls.

Overall spending on campaign ads this year is expected to exceed $3.3bn (£2.1bn), an all-time record for congressional elections. Most of this year's ads revolve around the economy and fears about joblessness, but there are many other telling themes.

Illegal immigration

Illegal immigration has been a touchy political topic in the US for some time. But with drug-related violence spiking in Mexico, anti-immigration forces have stepped up their rhetoric.

Media playback is unsupported on your device Media caption David Vitter's illegal immigration ad was condemned by Hispanic groups

Candidates in border states are particularly vulnerable to accusations of being weak on illegal immigration and border security. John McCain, once a leading voice for immigration reform, produced a widely-aired ad picturing him walking along the border with a law enforcement official and urging congress to "build the danged fence!"

In Alabama, Republican Tim James played on immigration fears in an ad where he said that if he became Governor, he would ban the practice of giving driver's licence tests in other languages.

"This is Alabama. We speak English. If you want to live here, learn it," he says. But Mr James failed to win the Republican nomination.

In Louisiana, incumbent Senator David Vitter has managed to divert discussions about his involvement in the DC Madam prostitution scandal, in part by turning the conversation to immigration.

Media playback is unsupported on your device Media caption Sharron Angle attacked Harry Reid over immigration

He ran a racially charged ad attacking his opponent, Congressman Charlie Melancon over illegal immigration. It features a group of shady, Mexican-looking men sneaking across the border and being met by white Americans bearing a giant cheque payable to "illegals" and a sign saying "Charlie Melancon welcomes you to the USA!"

The ad was condemned by Hispanic groups and bloggers.

Sharron Angle, the Tea Party-backed candidate challenging Senate Majority Leader Harry Reid has run similarly garish immigration ads, but in heavily Hispanic Nevada, they may have hurt her as much as they helped.

Both Ms Angle's ad and Mr Vitter's misrepresent the voting records of their opponents.

Fighting the last battle

Democrats started out this campaign season hoping voters would remember that President George W Bush was in charge when the US economy tanked in 2008.

Media playback is unsupported on your device Media caption Joe Sestak's ad may be the only campaign ad to have included a dog's business

But voters seem increasingly immune to references to the previous commander in chief. The country's economic woes are now laid squarely at Barack Obama's feet.

A few Democratic candidates have persisted, though, like Pennsylvania Senate candidate, Congressman Joe Sestak. After being attacked for voting for the Obama stimulus package, Mr Sestak ran an ad where he likens the early actions of the Obama administration in bailing out the economy to cleaning up after his puppy Belle. In other words, Mr Sestak and Mr Obama were cleaning up Mr Bush's mess.

It's hard to know if the ad had any effect, but it certainly got people talking about Mr Sestak, which may help explain his last-minute surge in the polls, though he remains the underdog.

Healthcare reform

Media playback is unsupported on your device Media caption Scott Murphy's advert attacks his rival on healthcare

When the process of reforming healthcare started in the early days of the Obama administration, some strategists believed that, come mid-terms, Democrats would broadcast ads trumpeting the successful passage of reform.

Instead, Democrats are giving a wide berth to "Obamacare", as conservatives have dubbed the bill. While Republicans attack the health plan and make campaign promises to repeal it, most Democrats remain mum on the subject.

But at least one brave Democrat has embraced "Obamacare". Scott Murphy, who is campaigning in New York, is running the sort of pro-health care ad Mr Obama probably dreams of.

Running against President Obama

Media playback is unsupported on your device Media caption Ben Quayle promises to 'knock the hell' out of Washington

Mr Obama's approval ratings have plummeted since he took office. Republicans have worked hard to make the election a referendum on the president's increasingly unpopular policies.

Few have been as bold in their criticisms, though, as Ben Quayle, son of former Vice-President Dan Quayle. In one campaign ad, Mr Quayle solemnly declared "Barack Obama is the worst president in history" and promised to "knock the hell" out of Washington.

However, having a different Democratic president - Bill Clinton - on your side appears to be a good thing this year.

Media playback is unsupported on your device Media caption Meg Whitman uses archive footage of Bill Clinton in her ad to attack rival Jerry Brown

Mr Clinton has been asked to campaign for Democrats across the country, and polls indicate that support from him does more to boost a candidate's prospects than support from the current president.

But one cheeky Republican is using Mr Clinton's resurgent popularity to hurt a Democrat.

Former eBay CEO Meg Whitman, who has carpet-bombed California's airwaves with more than $100m (£62m) worth of ads, is using clips from a 1992 presidential debate, when President Clinton attacked his then-opponent Jerry Brown - who is now running against Mrs Whitman for California's governorship.

Anti-incumbency vibe

"Washington is broken" is possibly the most common refrain of 2010's election season. Politicians across the country are seeking to distance themselves from the nation's capital and "politics as usual".

That makes being an incumbent difficult. And it makes being the leader of the Senate almost a poisoned chalice - just ask Harry Reid.

Media playback is unsupported on your device Media caption Republican-funded mid-term ad points out that Harry Reid lives at the Ritz Carlton when in DC

Mr Reid is locked in a tight race with Tea Party-backed Sharron Angle. Although Mrs Angle has made numerous campaign mistakes - including telling a group of Hispanic students they looked like Asians to her - ads tying Mr Reid to the "corrupt" political culture of DC have been brutally effective, creating a contest full of suspense.

In a powerful attack ad on Mr Reid, Republicans point out that while in DC he resides at the opulent Ritz Carlton hotel.

One of the most aggressive anti-incumbency ads, though, is being aired by a Democrat. Joe Manchin, the West Virginian governor running for his state's open Senate seat, literally loads up his gun and shoots a bullet through a climate change bill.

Viral ads

Media playback is unsupported on your device Media caption Carly Fiorina's 'demon sheep' attack ad went viral

Republican advertising guru Fred Davis, of communications firm Strategic Perception, adheres to a philosophy that the best political ads get people talking. And Mr Davis's ads certainly sparked chatter this year.

In 2008, he was responsible for the "Celebrity" ad, which juxtaposed Mr Obama with images of starlets like Britney Spears, while a breathy announcer intoned "he's the biggest celebrity in the world".

This year, Mr Davis created two of the most-discussed ads. The first involved a "demon sheep" - the primary opponent of California Republican senate candidate Carly Fiorina is pictured as a phoney, red-eyed wolf dressed up in sheep's clothing.

Media playback is unsupported on your device Media caption Christine O'Donnell tells voters she is not a witch, rather "I'm you"

The second opened with the unforgettable phrase: "I'm not a witch". Delaware Republican senate candidate Christine O'Donnell tackles her widely pilloried admission that she had "dabbled" in witchcraft at college by looking straight at the camera and declaring that far from being a witch, she is just like the average voter.

Demon Sheep was not shown on television, but it has been viewed more than 500,000 times on YouTube.

Still, ad-related chatter doesn't necessarily translate into votes. Mr McCain's presidential bid failed, despite the success of the Celebrity ad. And both Ms Fiorina and Ms O'Donnell trail their opponents in the polls.