It soon became clear, however, that Loughner had no apparent political motive and had suffered from mental problems. Yet Mrs Palin reignited the political row around the shootings by describing the allegations against her as a “blood libel”, in a broadcast credited by some commentators as starting a downfall that nixed the prospect of a presidential run.
Mr Obama hurried to the scene to visit Miss Giffords in hospital, comfort local people and to deliver a speech that scaled the rhetorical heights he had failed to climb since entering the White House, in which he called for a “more civil and honest public discourse”.
The plaudits he received for the speech were rare during a year in which his approval rating tumbled to 43 per cent – lower than Jimmy Carter's as he prepared for his doomed re-election campaign in 1979. Several economic indicators were so bad as to require Mr Obama to break post-war precedent if he were to manage to be re-elected.
Unemployment remained around nine per cent throughout the year, meaning joblessness had not been so high for so long since the Great Depression. The poverty rate climbed to its highest rate since 1983 and growth remained sluggish as fears of a double-dip recession persisted.
It was in this context that the Occupy Wall Street movement, which gave voice to millions disgruntled with the economic status quo, was born in September. it soon spread across the US and the rest of the world. Aides to Mr Obama indicated that he would seek to style himself as the candidate of the “99 per cent” in 2012.
But while he achieved a string of triumphs in foreign affairs – having the al-Qaeda leader Osama bin Laden and one of his senior disciples, Anwar al-Awlaki, assassinated and helping to oust Muammar Gaddafi from Libya – the president was repeatedly frustrated on economic and domestic policy.
A shutdown of the federal government was narrowly avoided in April after Mr Obama was forced to accept a $40 billion cut in public spending. A stalemate in December had been averted only after Mr Obama agreed to extend George W. Bush's ruinous tax cuts for another two years.
At the end of July, the US came close to defaulting on its debt when the two parties could not strike a deal to raise the $14 trillion debt ceiling – the limit on what the government can borrow – before the Democrats agreed to $2 trillion in spending cuts.
But in keeping with the year, a “super-committee” of senior congressmen, convened under the terms of the deal to come up with a plan for how the cuts should be made, failed to strike any deal whatsoever, meaning deep cuts to treasured areas including defence should be automatically triggered.
Yet another impasse looked likely at year's end, with Mr Obama threatening to make Congress stay at work through their Christmas holiday until an agreement could be made on extending a cut to the payroll tax, the US equivalent of national security, which he says would benefit middle earners.
Civil and labour rights returned to the agenda, with New York becoming the sixth and largest state to allow gay marriage and battles being fought in Wisconsin and Ohio over the power of trade unions. Candidates for the Republican presidential nomination promised to crack down on both.
Their campaigns, which were approaching their first test in the Iowa caucus as the year ended, dominated the last few months of the year, as one by one the candidates on the Right of the party enjoyed a spell as the leading contender to Mitt Romney, the former Governor of Massachusetts, and the party's heir apparent after losing in his bid for the nomination in 2008.
But the surges in support for Rick Perry, the Governor of Texas, Michele Bachmann, a Minnesota congresswoman, and Herman Cain, a former pizza restaurant tycoon from Georgia, eventually subsided, with the latter forced to withdraw following accusations of infidelity and sexual harassment.
By the end of the year it was the unlikely figure of Newt Gingrich, the foil to President Bill Clinton as Speaker of the House of Representatives in the 1990s, who managed to secure – and then maintain – polling leads over Mr Romney in three of the four states due to vote on the contest in January.
Admitting that he faced a tough re-election campaign in key states such as Florida and Ohio, Mr Obama told supporters in December: “We're going to win this thing”. As they prepared to have their say on a candidate to face him, millions of Republican activists ended 2011 determined that this would not be so.
Read more reviews of the year 2011