An estimated 10,000 to 12,000 protesters converged on the nation's capitol Sunday to press President Obama to block construction of the 1,700-mile Keystone XL pipeline, which would transport tar-sands oil from Canada to refineries on the Gulf of Mexico.  The rally, which culminated in a human ring encircling the White House, was a sequel to August's civil disobedience campaign, during which 1,252 protesters were arrested. By 1 p.m., throngs of ralliers -- wearing bright orange vests emblazoned with the words "STOP the Pipeline" -- were already congregating at Lafayette Square Park across from the White House. Protesters from across the …