By Moira Warburton WASHINGTON (Reuters) – Control of the U.S. Senate is at stake in the Nov. 8 midterm elections, as Republicans need to pick up only one seat to win control of the 100-seat chamber, which would allow them to block much of President Joe Biden’s agenda. The chamber is currently split 50-50 between Republicans and Democrats, who maintain control thanks to Vice President Kamala Harris’ tie-breaking vote. Democrats hope to expand their margin. Here are eight races that could determine the outcome. OHIO Republican J.D. Vance, author of the hardscrabble memoir “Hillbilly Elegy,” has …