Barack Obama will on Saturday embark on a whistle-stop train tour of Pennsylvania three days before the state holds a critical Democratic primary in a move redolent of Harry S. Truman’s successful 1948 campaign.

Mr Obama, who remains behind Hillary Clinton in Pennsylvania by between 1 and 20 percentage points among wildly gyrating poll numbers, will stop at several towns between Philadelphia and Harrisburg, the state capital, and “slow-roll” through many more.