Democratic presidential candidate Barack Obama, already leading in the polls, was expected to get a further boost Sunday when former President Bill Clinton and his wife, Sen. Hillary Clinton, begin making joint campaign appearances on his behalf. The nation's best known and most powerful Democrats for nearly two decades will be on the road for Obama, who vanquished Hillary Clinton last spring in a bitter primary contest. The Clintons apparently have put that behind them and will stump for Obama's election. The Clintons are to appear with Obama's running mate, Joe Biden, at a rally Sunday in the working class town of Scranton, Pennsylvania. The location is meaningful because Biden was born in Scranton and lived there for several years as a child, while Hillary Clinton's father grew up in the town and is buried there. On Saturday, Republican presidential candidate John McCain toned down his rhetoric against Obama, apparently concerned with angry sloganeering from supporters at some of his rallies - and criticism that he had gone too far.