If the US’s policy towards the Chinese currency had a theme song, Charles Schumer opined on Thursday, it would be the 1975 Maxine Nightingale classic “Right Back Where We Started From”.

The Democratic senator from New York has been pushing for tougher action against Beijing to force the authorities to allow the renminbi to resume the appreciation halted in 2008.

So far, there has always been enough countervailing force in the form of the administration of Barack Obama, the House of Representatives and the US business community to hold up his ideas of imposing penalties on Chinese imports.

But with each month that the long-awaited resumption of Chinese exchange rate flexibility does not happen, the bulwarks weaken against what is likely to be regarded in Beijing as the opening offensive of a trade war.

On Thursday, the beneficiary of Mr Schumer’s musical edification was Tim Geithner, the Treasury secretary, appearing in front of the Senate finance committee. A softly-softly approach from the US Treasury in recent months has failed to produce action from Beijing, partly because the fall in the euro has made China reluctant to act.

China-watchers have long taken the view that the Congress and the administration play a good-cop, bad-cop game with China.

Blood-curdling threats of confrontation from Capitol Hill, on which lawmakers never actually intend to follow through, are used by the more outwardly emollient US Treasury for leverage in its bilateral negotiations with Beijing. But on Thursday, Mr Geithner seemed to be hewing more closely to the aggressive congressional line, saying Mr Schumer’s repeated efforts over the years to bring currency legislation had had a beneficial impact on the debate and claiming ignorance over Beijing’s immediate intentions.

“I, to be honest, do not know whether we will see meaningful progress in the short term,” Mr Geithner said. “I think it is important everyone in China understands…that there is broad support in Congress for moving on this very important issue.”

Mr Schumer said this week that he was prepared to move his bill unless he saw action within two weeks – not coincidentally the period of time until the Group of 20 leading economies’ heads of government meeting in Toronto. He has identified a bill passing through the Senate about small businesses to which it can be attached.

Derek Scissors, a China expert at the conservative Heritage Foundation, said the degree of seriousness from Mr Schumer remained unclear, as did Mr Geithner’s strategy. But he said real pressure was undoubtedly building from Congress.

“Geithner may just be letting Congress yell throughout June because he knows they want to and that Beijing will move [on the currency],” he said. “But he might be genuinely through with playing good cop. He will be concerned about going out on a limb only to have China saw it off.”

The current incarnation of Mr Schumer’s bill is less aggressive than previous versions, which would have slapped an across-the-board tariff on Chinese imports. It would allow companies to use estimates of currency misalignment when calculating the “anti-dumping” and “countervailing” duties they can impose against imports deemed to be priced unfairly low or state-subsidised.

But being less combative and more likely to survive challenge at the World Trade Organisation, such a bill would also be harder for Mr Obama to veto without sustaining real political damage. “If a bill like that ever came to the Senate floor, who would dare to vote against it?” said Chris Nelson, author of a regular newsletter on Asia read widely in Washington. “At the moment we are still in the laying-down-markers mode, but you have to say they are running out of time.”