Over 2,000 delegates from the Democratic and Republican parties will be attending national party conventions over the next few weeks. Millions of voters are expected to view the televised events, which are meant to convey an image of party unity. Reporter William Eagle takes a look at how they have evolved since their beginnings nearly 180 years ago.

Today, some observers see the U.S. political party conventions as carefully choreographed exercises in public relations. They serve mostly to formalize the selection of the presidential nominee already chosen in party caucuses and primaries. And they showcase support for the official candidate and the platform, or program, which each party supports. But for many decades, the conventions were unpredictable ? and were known for compromises and last-minute decisions.

Julian Zelizer is a professor of history and public affairs at the Woodrow Wilson School at Princeton University.

"They are an invention of 19th century America," he says, "and before the 1970s, these were places where party delegates gathered to decide on their candidates for president and vice president. The delegates were often party bosses and operatives who decided who is a better candidate in terms of what the Republicans and Democratic parties represent."

Deal making could be long and difficult. In 1924, deadlocked Democratic Party delegates took more than 100 ballots before choosing a compromise candidate, John Davis, who then lost to Republican Calvin Coolidge.

At times, even tough bargaining could not unify a party convention. In 1860, a debate over slavery ended with the walkout of delegates from the south. They left because the party had adopted the position that the Supreme Court could decide whether to outlaw the practice, taking the decision out of the hands of the individual states.  

In the 1970s, both major parties enacted reforms to make the selection of the presidential nominee more democratic. Since then, the head of the ticket has been the candidate who has won a majority of delegates in caucuses and state primaries run by the party.

Usually, the elected delegates vote according to the wishes of their constituents, although the Democratic Party also has what it calls super delegates, who vote according to their own views. The super delegates are mostly current or former party leaders and elected officials. In the final days of the party's primaries this year, both Barack Obama and Hillary Clinton courted super delegates as a way to attract the votes needed to win the Democratic Party's nomination.

Although Senator Obama won enough delegates to secure the nomination, Democrats plan to place Senator Clinton's name in nomination. After acknowledging Senator Clinton's delegates, they would then be expected to give their full support to Obama. The symbolic gesture is seen as an attempt to unify the party after a closely fought primary season. Professor Zelizer says this still may not be enough for some activists.

"There's not a sense that Obama's victory could be upset," he says, "but there could be some kind of protest by [Clinton] supporters if she is not selected as vice president?there could be rallies around the convention or some kind of symbolic protest vote."

Convention unity can often be achieved by including the more ideological delegates in the creation of the parties' platforms, or programs. These delegates may not represent the views of the party's majority, but they are more likely to support the platform if they feel their views have been heard.

This was the case 33 years ago. Conservatives backing candidate Ronald Reagan disagreed with the moderate foreign policy of President Gerald Ford, who was running for re-election. Mr. Ford's policy included a relaxation of tensions with the Soviet Union and the handover of the operation of the Panama Canal to Panama.

"Reagan challenged the incumbent Republican president] Gerald Ford. [He] insisted on a morality in foreign policy plank [of the platform] -- criticizing the policies of [President] Ford, who was a moderate Republican. Reagan's people insisted on having that accepted," he says.

The demands were included in the platform.

Zelizer says today, delegates usually reach a consensus ahead of the convention, and television viewers see a show of harmony between the various party factions.

At this point, observers say, a public display of discord would be?unconventional.