Typically, the candidate who wins the most delegates becomes the party's nominee. The national Republican and Democratic parties allocate delegates to each of the 50 states based roughly, though not entirely, on population.

The state parties decide what voting system they will use. The most common system is the primary election. Voters go to the polls and cast a ballot for their candidate of choice as in any other election. Some states have open primaries, while other states have closed primaries.

In an open primary, any voter regardless of party affiliation can vote for the Democratic and the Republican candidates. In a closed primary, only Democrats can vote for Democratic candidates, and only Republicans can vote for Republican candidates.

There is another system called a caucus, and the most famous one is held in the Midwestern state of Iowa. In a caucus, party members meet at their local precinct and discuss their reasons for supporting different candidates before voting begins.

"What makes it impossible for Americans to understand, let alone someone from another country, is that each state is different in terms of the kinds of nominating rules that they might have," Robert Schmuhl, an expert on the presidential nomination process at the University of Notre Dame, told DW.

Early states key

The current nominating system was adopted in the 1970s, first by the Democrats and then by the Republicans, in effort to make the process more open and democratic. Previously, party elites had the biggest say in who became the Democratic and Republican nominees for president. Voters now have a much greater role.

Through this reform, Iowa became the first state on the calendar. The state's caucus system is complex and can drag on, so the decision was made to hold it as early as possible. The northeastern state of New Hampshire holds its primary shortly thereafter.

Since the 1970s, Democratic and Republican presidential candidates have built campaign strategies around Iowa, New Hampshire and other states that vote early. A strong showing can provide momentum in the long and often grueling race to the nomination.

In 1976, a relatively unknown governor from Georgia, Jimmy Carter, took the media by surprise and caused a sensation by winning the Iowa caucuses. He went on to become the Democratic nominee and win the presidential election.

In 2008, few expected Illinois Senator Barack Obama to beat New York Senator Hillary Clinton and become the Democratic nominee. The Obama campaign focused on the caucuses, while Clinton focused on the primary states. As consequence, he won Iowa and gained a burst of momentum.

"Whoever came in first, second or third in Iowa since 1972 has always gotten the nomination of their party in both the Democratic and Republican parties," Steffen Schmidt, an expert on the caucus system at Iowa State University, told DW.

'Americans are frustrated'

In the summer, the Republicans and Democrats hold their national party conventions, where all the delegates from all 50 states gather to select the party's presidential nominee.

"The Democrats have almost twice as many delegates to their convention than the Republicans," Schmidt said. "Because the Democrats had these party reforms that gave states more delegates in order to have women and minorities and youth votes and so on."

Some states have a winner-take-all system. The candidate who wins a plurality of the state's primary or caucus votes receives all of the state's delegates at the convention. Other states have moved toward a proportional system. Candidates receive delegates based on their percentage of the popular vote.

Most of the time, delegates select the nominee in the first ballot at the convention. But it could get messy this year. With so many candidates in the Republican field, a brokered convention is possible. In this scenario, no candidate emerges as the winner in the first ballot and horse trading goes on between the delegates to find a nominee.

In most states, only 20 percent of Americans bother to vote in the presidential primaries and caucuses, according to Schmidt.

"It's totally non-transparent, and it's no wonder that Americans are frustrated with politics because the process is just too complicated," Schmidt said. "Someone needs to come in there and say this doesn't work, people are alienated from politics because it's mumbo jumbo."