The major method used by the government to ensure public support of the war was propaganda. The US government set up a major propaganda apparatus whose job was to get people to support the war both through their opinions and by loaning money to the government.
The main part of the propaganda apparatus was a committe called the Committe on Public Information (CPI). The CPI produced many kinds of propaganda. These forms included posters with patriotic and anti-German messages. They also included the mobilization of a huge group of speakers trained to deliver propaganda messages to public groups. Other tactics included the creation of pamphlets and movies.
This sort of propaganda was the main way in which the government tried to win support for the war (as opposed to suppressing dissent).