After months of taking "extraordinary measures" to prevent a first-ever U.S. default, Treasury Secretary Janet Yellen on Monday warned that "our best estimate is that we will be unable to continue to satisfy all of the government's obligations by early June, and potentially as early as June 1, if Congress does not raise or suspend the debt limit before that time." Amid calls for a clean bill to raise the nation's arbitrary borrowing limit to avert a default that economists say could be catastrophic for the U.S.