HARRISBURG, Pa. — Democratic Sen. Bob Casey went head-to-head with Republican challenger Dave McCormick in a bitter first debate Thursday night that reflected the close nature and high stakes of a race that could help determine the balance of power in Washington.

The hourlong showdown touched on topics from the economy to abortion to energy — and it frequently got personal, as each candidate repeatedly tried to paint the other as a liar. Casey targeted questions about McCormick’s…