No one expected Campaign 2012 to be positive or uplifting. The country’s problems are too severe and the battle lines between Republicans and Democrats have been hardened by almost four years of conflict between the White House and Congress.

But what is most striking about the contest is not just the negativity or the sheer volume of attack ads raining down on voters in swing states. It is the sense that all restraints are gone, the guardrails have disappeared and there is no incentive for anyone to hold back. The other guy does it, so we’re going to do it, too.

Mitt Romney’s selection of Rep. Paul Ryan (Wis.) as his vice presidential running mate seemed like an opportunity for both sides to pause and reset after one of the ugliest weeks of the year. Instead, this week has produced the harshest rhetoric and the angriest accusations of the campaign.

Vice President Biden triggered the latest round Tuesday with lines that, had a Republican uttered them, probably would have set off an even bigger firestorm. He told an audience in Virginia that Romney would “unchain” big banks if elected and then added, “They’re going to put y’all back in chains.”

Biden later tried to temper his language, but the damage was done. Within hours, Romney unloaded on the president. Campaigning in Ohio, he said that Obama’s “angry and desperate” campaign had brought disrespect to the office of the presidency. “Mr. President,” he added, “take your campaign of division and anger and hate back to Chicago and let us get about rebuilding and reuniting America.”

That brought an incendiary response from the Obama team. Spokesman Ben LaBolt said Romney’s comments “seemed unhinged.”

Both Romney and Obama talk about this campaign being about big choices. That’s certainly true, given the candidates’ opposing worldviews. But fear and anger motivate each side’s activists. Partisans imagine the worst will happen if the other side wins. That, in turn, animates the strategies unfolding now.

Mock outrage has long been a part of every campaign’s toolbox, but there is a sense now that the outrage is genuine, that the disrespect that the Chicago and Boston teams feel for each other has escalated and becomes the justification for ever harsher attacks.

Neither side has had to look far for an excuse to attack or cry foul. Obama’s allies took the campaign over the edge last week and his team did nothing to stop it. The most egregious example of a campaign out of bounds was an ad prepared by Priorities USA, a super PAC supporting the president.

The ad linked Romney to the cancer death of the wife of Joe Soptic, who lost his job and health insurance when a steel company that Bain Capital took over while Romney was at the firm later went bankrupt, after Romney left Bain.

The spot was not shown on television last week but did air in Cleveland this week. Obama campaign advisers at first tried to distance themselves from it by saying they didn’t know the details of Soptic’s situation. In fact, they had used him in an ad earlier this year and put him on a conference call with reporters at the time.

The Obama campaign also has declined to denounce Senate Majority Leader Harry M. Reid (D-Nev.) for making the unsubstantiated accusation that Romney paid no taxes for 10 years. He said that a Bain investor told him that, but he would not identify the person or retract the claim when Romney denied the charge.

Mention the Soptic ad to Obama campaign officials and instead of showing remorse or regret, they point to the spot Romney aired that accuses Obama of gutting the work requirement in the welfare reform act that was passed by a Republican Congress and signed into law by President Bill Clinton in 1996.

The changes were in response to requests from some governors, including Republicans, who wanted more flexibility. Administration officials say they are not letting states off the hook on the work requirement, and Clinton denounced the ad as false. A leading Republican welfare reform expert has said it is “implausible” to believe that Obama is trying to keep more people on welfare. Fact-checking outlets have declared the ad erroneous. Romney’s campaign has doubled down rather than walk away.

Negative ads have become one of the growth industries in an otherwise weak economy. How much is being spent? Romney’s campaign briefed reporters last Friday and included the following statistics. The amount spent on all advertising since early April in four key states is: Florida, $95 million; Ohio, $92 million, Virginia, $68 million; and North Carolina, $50 million.

News organizations instituted fact-checking and ad watches in reaction to earlier campaigns, when candidates were getting away with half-truths and worse, with little accountability. These have become robust and increasingly comprehensive. But they are not providing much of a check on the campaigns’ behavior.

The only check on the campaigns is the marketplace, said John Geer, a political science professor at Vanderbilt University. “If voters move against his attacks, [Obama] will move away from them,” he wrote in an e-mail response to a question. “But right now, the attacks are working on swing voters. The other 90 percent of the public are pretty much fixed in their preferences. They may be unhappy about [the ads], but they are not driving the marketplace.”

But there is no check on rhetoric. Romney and his advisers have been seething over the tactics of Obama’s campaign and its Democratic allies, including the Soptic ad and the president making what seemed like a joking reference Tuesday to an old story about Romney strapping his dog to the top of the family car during a vacation.

Obama and his team have their list of grievances about the claims and accusations made by Romney and his allies. They point to what they view as rhetoric questioning the president’s patriotism and American values — code, they believe, for a revival of birtherism.

This campaign will end in November. Then it will be either Obama’s or Romney’s responsibility to try to govern. Both sides have turned the contest into an all-or-nothing battle and hope to claim a mandate on the basis of the outcome. But it will take time and great effort for the winner to drain the poison from the system if the campaign continues on this course.