WASHINGTON, Nov 4 (Reuters) - Democrat Barack Obama won a big victory over Republican rival John McCain on Wednesday in Pennsylvania, one of three critical swing states that have played a decisive role in presidential elections for the past 50 years, U.S. television networks projected.

Obama had been expected to win Pennsylvania, with several opinion polls before the vote giving him a double-digit lead. Along with Ohio and Florida, Pennsylvania has had a major role in presidential elections for the past half century. No candidate has won the White House without capturing two of the three states since 1960.

The victory in the sixth most populous U.S. state gave Obama Pennsylvania’s 21 electoral votes, a big step toward the 270 needed to take the presidency in the United States’ indirect system of choosing a leader.