By Steve Schifferes
Political conventions have played a unique role in American political history.
Barack Obama made a mark with his 2004 convention speech
In theory, the convention is a conference where party activists come together to choose a presidential nominee.
But in recent years, the parties have managed to narrow down their choice to just one candidate long before the convention begins.
Consequently, they have become stage-managed events, marking the official launch of the presidential campaign of a candidate already selected in the primary contests earlier in the year.
In the past, contested conventions have changed the course of American history - and with the two Democratic candidates running neck-to-neck, their party's convention in August may do so again.
The bitterly contested 1968 convention damaged the Democrats
Conventions can reveal the disarray within political parties - most notably, at the Democratic convention in Chicago in 1968.
When anti-war demonstrators clashed with police outside the convention centre, it damaged the prospects of the party's nominee Hubert Humphrey.
He went on to lose the close-run contested general election to Republican Richard Nixon. The violent scenes prompted the Democrats to change their rules to give more power to primary delegates.
Conventions can also lay bare ideological rifts within a party.
The booing of moderate Republican candidate Nelson Rockefeller in 1964 in San Francisco foreshadowed a shift in the Republican Party to the right.
The convention nominated the conservative Barry Goldwater, who lost heavily to Lyndon Johnson.
But in 1984 he returned triumphantly to the Republican convention in Dallas to deliver a valedictory speech supporting President Ronald Reagan - his ideological successor - for re-election.
A deadlocked convention is usually bad news for a political party.
FDR became the first candidate to appear in person at a convention
In 1924 the Democratic convention in New York's Madison Square Garden was split over alcohol prohibition.
The southern wing of the party favoured William G McAdoo - a "dry" - while the northern wing backed New York governor Al Smith, a supporter of the repeal of the ban on the sale of liquor (a "wet").
After 103 ballots, a compromise candidate, John W Davis, emerged.
He was soundly defeated by Republican Calvin Coolidge, despite a financial scandal involving the previous Republican administration.
At that time, a Democratic candidate was required to win a two-thirds majority to gain the party's nomination, a rule that was only abolished in 1936.
Conventions can also signal unity and the desire for political change.
In 1932, Democratic candidate Franklin Roosevelt became the first to appear in person at the convention to accept his nomination.
It was where he first launched his famous economic recovery plan - known as the New Deal - saying "I pledge you, I pledge myself, to a new deal for the American people."
Facing an opponent damaged by the Great Depression, the most severe economic downturn in US history, Mr Roosevelt won a landslide victory, and went on to win re-election three more times..
FDR was already the front-runner when he made his 1932 speech, but earlier conventions did allow long-shot candidates to use their eloquence to win their party's nomination.
Abraham Lincoln mobilised his supporters
This was the case in the 1860 convention of the newly organised Republican party, which unexpectedly nominated dark horse Abraham Lincoln as its presidential candidate.
The convention was held in Chicago's Wigwam Hall, and Mr Lincoln's party managers flooded the hall with his supporters, defeating favourite William Seward after several ballots and some backroom promises of Cabinet posts for Mr Lincoln's beaten opponents.
Mr Lincoln's enthusiastic backers then led him to a sweeping general election victory against a Democratic party divided over slavery.
The Cross of Gold
Thirty-six years later, in Chicago, the Democratic Party convention was in turn mesmerised by a young orator from Nebraska, William Jennings Bryan, who advocated "soft money" to help struggling farmers in the Midwest.
His attack on the gold standard, and his inspiring closing remarks, persuaded delegates to abandon their support for the sitting Democratic president, Grover Cleveland.
But in the end, Mr Bryan led the Democratic Party into the wilderness. Although he was twice again nominated, the Republicans won every presidential election until 1932, except in 1912 when the Republicans were split, letting in Woodrow Wilson (who also won narrowly in 1916).
1940 Republican Surprise
The political convention did not lose its ability to surprise in the 20th Century.
Wendell Willkie unexpectedly won the 1940 Republican nomination
In June 1940, the Republican establishment was stunned when its Philadelphia convention selected an unknown lawyer, Wendell L Willkie, to run against Mr Roosevelt in his bid for an unprecedented third term.
Mr Willkie, a corporate lawyer who had never run for public office before and had only been a Republican for two years, defeated party grandees Robert Taft and Thomas Dewey on the sixth ballot after tumultuous floor demonstrations from his supporters.
Mr Willkie's popularity was based on his opposition to Republican isolationalism, and his support for Britain at a critical stage in World War II, when Germany had just defeated France.
This defeat of isolationism was crucial for the survival of Britain, as it allowed the sitting president, Franklin Roosevelt, to let the US act as the "Arsenal of Democracy", providing arms and aid to the UK before it actually entered the war.
If the Democratic party hold its convention this year without a clear favourite, history shows there are both risks and rewards.
A bitterly divided convention could demoralise the party and energize the opposition.
On the other hand, a breakthrough victory - or a surprise twist - could inspire the Democratic electorate and give their campaign political momentum.
In either case, it could be the most interesting convention in a generation.