It sometimes seems if only three things are certain in life — death, taxes and the never-ending debate over the minimum wage.

Look for the issue once again to take center stage during the election year of 2016. Two candidates, Hillary Clinton and Bernie Sanders, have endorsed a $15-an-hour minimum wage for certain workers. A number of cities, however, are not waiting for Congress to act and are instituting higher wage floors of their own.

"It certainly has become a larger issue, because the SEIU (Service Employees International Union) has invested quite a bit of money into the effort," says Michael Saltman, research director for the Employment Policies Institute in Washington, D.C. "With the amount of money being spent, it's not surprising it has a high media profile. Their strategy is to go to cities where they know legislators are friendly to unions, such as Los Angeles and Seattle. The SEIU is pursuing a low-hanging-fruit strategy."