The U.S. national debt now exceeds 100% of gross domestic product, crossing a once-unthinkable threshold, on the way toward breaking the record set in the wake of World War II. As of March 31, the country’s publicly held debt was $31.265 trillion, while GDP over the preceding year was $31.216 trillion, according to data released Thursday. That puts the ratio at 100.2%, compared with 99.5% when the last fiscal year ended Sept. 30. That figure will likely climb for the foreseeable future because the federal government is running historically large annual deficits of nearly 6% of GDP, which add to...