The U.S. election polls have finally opened for voting in one of the most interesting presidential battles in history with Democrat candidate Hillary Clinton and Republican candidate Donald Trump going head to head as the 45th president of the United States.

As Clinton and Trump fight it out, either one of them has to gather 270 Electoral College votes to be declared as the victor. The U.S. Electoral College is the body given the power to elect the President and Vice President of the nation every four years. Although citizens do not directly elect their presidential candidate, they can choose “electors”, who pledge their vote for a specific candidate.

According to The Guardian, each U.S. state’s population serves as a basis for the assignment of a certain number of electoral votes. On average, a state is awarded one electoral vote for every 565,166 people.

For example, the state of California is assigned 55 electoral votes since it has the highest population while Texas gets 38, Florida gets 29, and New York gets 29. On the opposite end of the spectrum, states such as Alaska, Montana, and Wyoming get three electoral votes each.

Despite there being 50 states, most are considered either Republican or Democrat considering the voting history going back at least six elections. Louisiana is considered “safely Republican” while Washington is considered “safely Democratic.”

However, states such as Florida, Ohio, Colorado, Nevada, Iowa, North Carolina, Virginia, and New Hampshire have had a history of voting for either Republican or Democratic candidates in the past years, deeming them “swing states.” These are considered the most critical when it comes to campaigns.

The United States held its first ever presidential election in 1789, following the ratification of the United States Constitution in 1788. According to the U.S. National Archives and Records Administration, George Washington of the Federalist Party won the presidency in this election.

In the First Party System, the Federalist Party and Thomas Jefferson’s Democratic-Republican Party were of significance. In the Second Party System, the Democratic-Republican Party was split during the election of 1824, which saw a tight race between winner Andrew Jackson and John Quincy Adams.

The modern Democratic Party was eventually formed in support of Jackson in the rematch of 1828. With no other major candidates appearing in the race, Jackson maintained his win over Adams (National Republican) with 178 electoral votes.

Following the disintegration of the National Republican party, the Whig Party and Free Soil Party formed but also disintegrated. Finally, the Republican Party emerged in 1854 from the loose alliance of the two as well as anti-slavery activists.

The Third Party System, from 1854 until the mid-1890s, saw the rise of the Republicans as the dominating faction. Elected Republican candidates include Abraham Lincoln from the 1864 election, Ulysses S. Grant from the 1868 election, and James Garfield from the 1880 election.

The Fourth Party System continued to be dominated by the Republicans with the exception of Democrats Grover Cleveland and Woodrow Wilson elected into office. The Fifth Party System was then established in 1932 and the Democrats finally dominated. Franklin D. Roosevelt dominated the presidential elections from 1932 through 1944.

Democrats continued to dominate through the mid-1960s with the exception of Republican Dwight D. Eisenhower, who was elected for two terms from the 1952 and 1956 elections. Although the two parties continued to claim dominance in the following years, no one party has been able to hold the presidency for over three consecutive terms.

Other notable landslide victories through the years include Democrat Lyndon Johnson’s win in 1964, Republican Richard M. Nixon’s win in 1972, and Republican Ronald Reagan’s wins in 1980 and 1984.

The 2000 election was a close one with Republican George W. Bush’s win over Democrat Albert Gore, Jr.’s – 271 votes over 266. Despite this, Al Gore won the popular vote with 50,996,582 votes.

In 2008, Democrat Barack Obama defeated Republican John McCain with 365 votes. Incumbent President Obama, and his running mate, Vice President Joe Biden, were elected to a second term on the most recent 2012 presidential election.

The United States presidential election of 2016 is being held today on November 8, 2016 and is the 58th quadrennial presidential election.