By Brendan O’Brien and Rich McKay CHICAGO (Reuters) -Tens of millions of Americans were in the pathway of dangerous and damaging weather conditions as snowstorms moved across the Northwest and Midwest, flooding threatened the East Coast and potential tornadoes were on tap in the South. Airlines delayed more than 7,600 flights across the U.S. on Friday, including planes grounded at Chicago’s O’Hare International Airport due to winds and blinding snow. In Iowa, Republican presidential candidates canceled events three days out from the state’s caucuses, the first of the state-by-state contests in…