Cast your minds back to November. Barack Obama had received his "shellacking" in the midterm elections, as the Republicans regained a majority in the House of Representatives and seized control of 29 of the 50 state governorships. It was the worst midterm defeat for the Democrats since 1938. Just a week earlier the president's approval ratings had fallen to a record low of 37%.

Fast forward six months, and the president is enjoying the "Bin Laden bounce". His approval ratings stand at 52%, according to Gallup – up six points on April. Historians may look back on 1 May 2011, and the killing of Osama, as the day Obama secured his re-election.

But even before the al-Qaida leader was dumped in the ocean, Obama had reason to be optimistic. Just 18 months away from the next election he has no obvious or credible Republican opponent. So far, the listless lineup of potential presidential candidates resembles the characters from the bar scene in Star Wars – a motley collection of far-right loons, freaks and conspiracy theorists.

There's the former senator, Rick Santorum, who once compared homosexuality to bestiality and paedophilia; former vice-presidential candidate Sarah Palin, who has said America must stand with "our North Korean allies"; Congresswoman Michele Bachmann, who believes carbon dioxide is "not a harmful gas, it is a harmless gas"; former governor Mitt Romney, who has said he won't appoint Muslim-Americans to his cabinet; Tea Party Congressman Ron Paul, who wants to scrap income tax and abolish the education department; and former House speaker Newt Gingrich, who published a book last year titled To Save America: Stopping Obama's Secular-Socialist Machine. Oh, and the "birther" billionaire Donald Trump.

The heart sinks. Lamenting the presidency of George W Bush, the late JK Galbraith once remarked: "I never thought I would yearn for Ronald Reagan." The current Republican presidential field makes one yearn for Dubya.

The tragedy is that Obama needs to be held to account – but from a leftwing, not rightwing, direction. He has embraced and affirmed a centre-right world view utterly at odds with his 2008 presidential campaign, with its promises of "change", "reform" and a decisive break from the Bush-Cheney era.

Consider his record: he failed to close the prison camp at Guantánamo Bay; approved the use of military tribunals for detainees; "surged" 40,000 troops into Afghanistan; doubled the size of the detention facility at Bagram airbase; doubled the number of drone strikes inside Pakistan; gave CIA torturers immunity from prosecution; continued extraordinary rendition; said he didn't "begrudge" bankers paying themselves multimillion-dollar bonuses' ruled out a government-run "public option" on healthcare; froze pay for public sector workers; signed off on tax cuts for billionaires; vetoed a UN resolution condemning illegal Israeli settlement-building; and joined China in sabotaging the climate summit in Copenhagen.

Liberals have given Obama a pass. Some avert their gaze; others proffer excuses. He needs more time, they say. But he has had 29 months in office. He is a good man in a bad world, they say, before blaming the Republicans for all America's ills. But it wasn't a Republican Congress that forced him, for instance, to double the size of the Bagram facility – where human rights groups have documented torture and deaths – and deny prisoners the right to challenge their detention. He did that on his own. Bagram is Obama's Guantánamo.

The double standards are glaring. Imagine, for a moment, the outcry from Democrats if Dubya had held the 23-year-old US soldier, Bradley Manning – the alleged WikiLeaks source – in conditions described as "degrading and inhumane" by more than 250 eminent legal scholars. Shamefully, however, Obama publicly defended Manning's detention, including his solitary confinement, as "appropriate".

The irony is that Obama, a self-styled conciliator and healer, has spent much of his presidency appeasing Republican foes on Capital Hill and capitulating to corporations and Wall Street banks. He has eschewed populism, allowing the Tea Party to surf public anger over bank bailouts and bonuses, job losses and home repossessions.

But what else should one expect from a White House stuffed with corporate-friendly, Clinton-era figures? The president's chief of staff, William Daley, appointed in January, is a former banker, and opposed Obama's healthcare reform. His treasury secretary, Tim Geithner, George Osborne's new best friend, was one of the architects of bank deregulation. Meanwhile, progressive economic voices like Joseph Stiglitz and Paul Krugman are studiously ignored.

Obama hasn't just neglected his base, he has abused it. The president's former chief of staff, Rahm Emanuel, dismissed liberals who objected to Obama's healthcare bill as "fucking retarded"; the White House press secretary, Robert Gibbs, ridiculed the "professional left" and said liberal critics of the president "ought to be drug-tested". Obama himself has described Democrats opposed to his compromises on tax cuts as "sanctimonious".

I have a proposal. Why not give him an electoral target for this animosity? Why not run a left candidate against Obama in the Democratic primaries next February? A Democratic opponent would act as a countervailing force to whichever Tea Party-backed Republican he ends up facing in the presidential election. It might force Obama to triangulate to the left as well as the right, and encourage the Democrats to have a long-overdue discussion about their values, policies and direction.

An Associated Press poll last October found an astonishing 47% of Democratic voters believed that Obama should be challenged from within the party for the 2012 nomination. Potential candidates include Dennis Kucinich, Ohio's leftwing Congressman; Howard Dean, the populist ex-governor of Vermont; and Rachel Maddow, the cable news presenter. None of them would win. But that wouldn't be the point. It would be about holding Obama's feet to the fire.

It is a risky strategy, given that none of the last three presidents to face primaries while seeking re-election – Gerald Ford, Jimmy Carter and George HW Bush – survived to serve a second term. Would a primary challenge from the left wreck Obama's chances of re-election? I suspect not, given the Bin Laden bounce and the weakness of his Republican opponents. The question that progressives should ask is whether they believe Obama should only have to answer to the likes of Donald Trump and Sarah Palin.