WASHINGTON (AFP) - Republicans pinned America's "crushing" fiscal burden squarely on President Barack Obama on Tuesday after the national debt passed $16 trillion just before the Democratic National Convention opened.The US debt stood at $10.6 trillion when Obama took office in 2009, but by implementing an economic stimulus plan aimed at heading off a depression the figure has soared - negating the president's pledge to halve it by 2012.Total outstanding US public debt hit $16 trillion late Friday, but the Treasury only released the statistic on Tuesday, after a holiday-weekend.The Republican National Committee and the campaign of Mitt Romney, the man challenging Obama for the White House in November's election, released a one-minute video highlighting Obama's own pledge to cut the debt in half by the end of his first term.And it heaped pressure on his administration to explain the federal spending that has added around $5-trillion to the debt since Obama took office. House Speaker John Boehner said the news marked "another sad reminder" of Obama's broken promise to slash government spending."Instead of working in a bipartisan way to fulfil his promise, the president went on a 'stimulus'-fuelled spending binge that stuck every American man, woman and child with a $50,000 share of this $16 trillion national debt," Boehner said in a statement."This debt is a drain on our economy and a crushing burden on our kids and grandkids, and it's yet another indication that the president's policies have made things worse."Republicans have hammered Obama on multiple fronts this week as he prepares to accept on Thursday his party's nomination for re-election.First they asked an age-old US election campaign question: are voters better off now than they were four years ago? Then they pounced Tuesday when Obama gave himself a grade of "incomplete" on fixing the economy.In their web video, the Republicans showed clips of Obama as a candidate in 2008, slamming the $4-trillion rise in national debt under president George W. Bush as "irresponsible" and "unpatriotic.""If we keep on adding to the debt, even in the midst of this recovery... at some point people could lose confidence in the US economy in a way that could actually lead to a double-dip recession," Obama says in the clip."By his own measurement, President Obama has failed," according to the video's text.