An electoral mandate is the permission granted to a political leader or winning party by the constituency to govern and act on their behalf. The mandate is more or less in effect for as long as the government is in power.
The American people, for example, are said to have given President Barack Obama an electoral mandate because they elected him to office. The people who elected President Obama to office voted for him because they supported his policies and approved of the decisions he said he would make as president. However, there is no formal, precise way to give President Obama, or any president or political leader, a mandate, so voters sometimes disagree on whether a leader has actually received such a nod from the public.
In the 2012 presidential election, President Obama won with 332 electoral votes but, at the time at which he was declared the victor, he had won only 50.3 percent of the popular vote. Some political commentators questioned whether he had actually won an electoral mandate after winning the popular vote by such a narrow margin. By stating that he did not, opponents implied that he had not won enough votes to earn the authority to act on behalf of the American people. Rhetoric aside, it's a common belief that all presidents, not just President Obama, must have popular support from the American people before attempting to realize big policy changes.
Although electoral mandates are sometimes a matter of dispute, and the term can be used for rhetorical purposes, their occurrence, and the belief that anyone with such a mandate should act on behalf of all voters, is generally linked to winning a democratic race, regardless of the margin by which the contest was decided.