The United States was today pushed to the brink of an unprecedented debt default as talks to end two-week-long shutdown collapsed. This prompted a top ratings firm to warn of a possible downgrade in the country's creditworthiness, which will have a cascading impact on the global economy.Frantic attempts to persuade House Republicans to lift the federal debt limit collapsed today. This left Washington veering toward a critical tomorrow deadline to raisethe country's debt ceiling.House speaker, John Boehner failed to persuade elements within his own Republican caucus to back a billdesigned to placate conservatives.Boehner's bill, which was drafted after Tea Party-backed members of the House voiced their opposition to a deal forgedbetween Democrats and Republicans in the Senate, had been expected to be put to a vote last night.But it was pulled at the last minute after the House leadership realised that the bill, opposed by Democrats, didnot even have the support of sufficient Republicans to pass.