US billionaire Warren Buffett's famous observation that he pays less tax than his secretary gave Barack Obama the political opening he needed to shift the tax debate to the left. The US President proposed a minimum tax rate of 30 per cent for those earning over $1 million (£630,000) a year, a measure that became known as the "Buffet Rule" (the inspiration for Nick Clegg's "tycoon tax").

But last night the resultant bill - the Paying a Fair Share Act 2012 - was blocked after the Senate voted not to open debate on it. The chamber backed the measure by 51 votes to 45 but this was nine short of the 60 votes the Democrats neeeded to overcome a Republican filibuster. In total, forty four of the 47 Senate Republicans voted against a debate, with just one - Senator Susan Collins of Maine - voting in favour.

Obama responded by arguing that "It's just plain wrong that millions of middle-class Americans pay a higher share of their income in taxes than some millionaires and billionaires." But he will be comforted by the knowledge that it is the Democrats, not the Republicans, who are on the right side of public opinion. A CNN poll published yesterday showed that 72 per cent of the US electorate, including 53 per cent of Republicans, support the Buffet Rule. As Charles E. Schumer, the Democratic Senator for New York, observed, for the first time in decades, the GOP is on the defensive on the signature issue of taxation.

Uncomfortably for the Republicans, their presidential candidate of choice - Mitt Romney - is a major beneficiary of the current system. As his 2011 tax return revealed, he paid an effective rate of just 15 per cent on an income of $20.9m. Obama paid 20.5 per cent on an income of $789,674, a lower rate than many US voters but higher than Romney. Moreover, the President can point to the fact that he is at least trying to solve the problem. As the White House noted:

Under the president's own tax proposals, including the expiration of the high-income tax cuts and limitations on the value of tax preferences for high-income households, he would pay more in taxes while ensuring we cut taxes for the middle class and those trying to get in it

Unsurprisingly, then, Romney is under pressure to release his tax returns for before 2010. The longer the Democrats can keep this story on the front pages, the better for Obama. Romney may quip that the Buffet Rule would only raise enough to fund the government for 11 hours but he should never underestimate the symbolic force of taxation.