Key issues in the US presidential campaign

Probably no other law divides Republicans and Democrats as much as President Obama's Affordable Care Act. It makes health insurance mandatory for most Americans and has reduced the number of uninsured citizens. While many Democrats would like to expand what has been called Obamacare, most Republican presidential candidates oppose it and have vowed to abolish it if they get elected.

The terrorist attack in San Bernardino in which 14 people were killed has reignited the debate about national security and the fight against terrorism. Republican presidential candidates have criticized President Obama for being too weak in the fight against terrorism. One Republican presidential contender, Donald Trump, even advocated banning Muslims from entering the United States.

Trump also wants to keep Mexicans out of the US and has called for building a wall along the US-Mexican border. Other candidates are more liberal and favor a new immigration law to give the 11 million undocumented immigrants a perspective in the United States. With the growing electoral power of Hispanics, immigration is a key topic in the presidential election campaign.

Whether it is in an African-American church in Charleston, a primary school in Sandy Hook or in a movie theater in Aurora - mass shootings have become a regular occurrence in the US. Near the end of his term, Obama is now trying to tighten gun control laws in the country. But it is a polarizing issue. Many Americans strongly oppose any new measures that would restrict access to guns.

It is an ongoing debate in the US: Pro-choice or pro-life - for or against the right to an abortion. In particular, many conservative, religious Americans consider abortion a grave sin. Most Republican candidates oppose abortion while the pro-choice movement fights for the right of women to legally have an abortion.

Wealth in the US is distributed extremely unequally. Californian researchers calculated that the richest 1 percent of Americans own as much as the bottom 90 percent. It thus seems unlikely that the US will ever have a social welfare system like Europe. But with Bernie Sanders, this year's campaign has a candidate with a strong focus on wealth and income inequality issues.