Democrats gather at their national convention July 26 in Boston to officially nominate Massachusetts Senator John Kerry as the party's candidate for president. Republicans will do the same for President Bush at their convention in New York City at the end of August. Political conventions have been a tradition in the United States since the 1830s. But, the role of the modern convention is far different from what it was even 50 years ago.

For more than 100 years, party conventions actually selected the presidential candidates.

But all that changed beginning in the 1960s. Party activists and voters demanded more of a role in the selection of presidential nominees. From that point on, presidential contenders won their party's presidential nomination by accruing delegates through primary and caucus elections.

"So, conventions no longer nominate candidates," says Allan Lichtman, a presidential historian at the American University in Washington. "Instead, today, conventions set party rules. They write the party platform, and, most critically, today, conventions give a big boost to a campaign. They launch the campaign, and if it works well, give the candidate a big bounce upward in the polls."

That kind of convention boost depends on what happens at the convention. In 1968, the Democratic Party was torn apart by the war in Vietnam. Thousands of anti-war protesters clashed with police in the streets of Chicago.

Those images gave voters the impression of a party spinning out of control. That November, Democrat Hubert Humphrey was narrowly defeated by Republican Richard Nixon.

Washington-based political analyst Stuart Rothenberg says modern conventions emphasize party unity and the candidates. "These are now media events," he says. "They are opportunities for the presidential candidate, not for the party, for the presidential candidate to showcase himself and his ticket, and to deliver the message that he wants that will ultimately be the general election message."

Successful presidential candidates have used the party conventions to consolidate their own base of support, and, at the same time, reach out to more moderate voters in the middle of the political spectrum.

Bill Clinton did it at the 1992 Democratic Convention, and George W. Bush followed suit at the Republican Convention four years ago.

"Well, successful conventions actually tame the activist wings of their party," says historian Allan Lichtman. "Successful conventions do not come across as dominated by zealots, who are out of the mainstream. Today's conventions tend to be scripted very carefully, and they do tend already to appeal to the center of the American electorate."

The Democrats say the theme of this year's convention will be for a country that is stronger at home and respected abroad.

Republicans will emphasize President Bush's leadership in the war on terror and his stewardship of the improving U.S. economy.