The most important political party conventions in U.S. history, arguably, occurred in 1860, when the Democratic Party split into two over slavery and Republicans selected Abraham Lincoln on the third ballot. In the months that followed, the country headed toward secession and war.

Sen. Richard M. Nixon of California arrives at the Republican National Convention in Chicago in 1952 to be selected as the Republican vice presidential candidate. (AP)

There's been nothing nearly as dramatic since, especially in the modern era. Nowadays, the nominees are known well in advance, picked in primaries and caucuses — they're no longer selected in those proverbial smoke-filled back rooms. Most of the speeches and events are scripted. (With the exception of Clint Eastwood, perhaps.) Conventions have mostly become three-day commercials for the two major political parties.

And yet, says Julian Zelizer, a presidential historian at Princeton University, party conventions are still extremely important — and they can often change the course of political history in unexpected ways. We talked by phone recently about the long history of conventions, why they still matter in this age, and what the future might hold.

Brad Plumer: We're no longer at the point where presidential nominees are actually selected at the party conventions. So could it be fair to say that these things no longer matter?

Julian Zelizer: Yeah, for a long time before the 1960s they were actually the places where the candidates were selected, where party bosses still mattered, where there was an opportunity to question and challenge the different candidates. That changed in the late 1960s and early '70s, with the whole idea being that primaries and caucuses would allow non-party bosses to decide who the nominee was. So now they mainly just serve the function of showcasing the presidential and vice presidential nominees and the party. These are tightly scripted commercials. But that's still valuable. It's a moment when more people are watching, when the parties can explain what their candidate is about for more than 30-second spots.

The other place where I think conventions still have value is that they're a test of the nominees. One important thing that presidents do is speak. We're looking for someone who can give a very good speech, during a military emergency or an economic crisis. There's something to seeing Mitt Romney and Barack Obama step into the spotlight and have to do something that's a little more lengthy than a quick response to the reporter. We see what they look like, what they sound like. Voters get some sense of what this person can be.

BP: So let's talk about some of the more important speeches that have helped do this.

JZ: One of the more politically smart convention speeches came from Ronald Reagan in 1980. He was facing a Republican Party that was far more divided than it is today. You had social conservatives, neoconservatives, budget hawks. And so in his convention speech, Reagan very skillfully brought a lot of these things together. You could see in his speech how he was going to hold this coalition in the fall, how he would defeat the Democrats. He really laid out the themes for his presidency.

This is going back further, but Franklin Roosevelt did something similar in his acceptance speech in 1932. He introduced the New Deal, he started to outline how government would start intervening in agriculture and the urban crisis and unemployment and used it as a road map. So whereas Reagan used his speech to hold his political coalition together, FDR actually outlined his governing agenda.

Then in 1988, George H.W. Bush gave a very effective speech at the convention. He was seen as a weak candidate, but he gave a very powerful speech that helped him defeat Michael Dukakis in the fall. Of course, that speech also contained the promise not to raise taxes, which came back to haunt him when he signed a tax deal two years later. So that speech turned out to matter a great deal — it was held up as proof that he had betrayed conservative ideals.

BP: And there are plenty of people who have given important speeches even if they never went on to become president. Barry Goldwater in 1964, for instance.

JZ: Absolutely. In 1964, Senator Barry Goldwater makes his speech about extremism being a virtue and about his party returning to conservative principles. It's a disaster for that election. But it became inspirational, and the speech was absolutely crucial in the GOP's move to the right.

The same goes for 1948. This is still in the pre-scripted era, but Hubert Humphrey is an unknown guy running for Senate for Minnesota, and at the Democratic convention in Philadelphia, he calls on his party to support civil rights. He tells all the Southern delegates that the time has come to support legislation, that they can’t keep talking about states' rights. That was seen as an important moment when liberal Northerners within the party finally stood up on the issue. And some Southerners ended up bolting the party that year, led by Strom Thurmond.

And then one last one was Ted Kennedy in 1980. He lost the nomination to Jimmy Carter, but he still makes an impassioned speech about the Democratic Party and the need not to move to the center, he talks about the traditions of the party and sailing against the wind. Defend common people from business. For many liberals that was a real rallying cry, it gave them juice for the decade to come.

Plus the conventions can help showcase young talent in the part. The obvious example is 2004, when Barack Obama gave the keynote address and introduced himself to the country. He was unknown at that point, and the speech generated enough excitement that it ended up changing the course of presidential politics.

BP: So where did Romney's acceptance speech on Thursday fit into this? Are there any good historical analogues?

JZ: I thought it was a good speech for him. He’s not Ronald Reagan, he's not Ted Kennedy, he's not bringing that kind of firepower. It's not like Goldwater laying out a dramatic argument for where the party needs to go. The best analogy might actually be Bill Clinton in 1996, where the one thing Clinton did with his acceptance speech was try to offer voters a sense for why he can win moderate votes, why he represented the new political middle. Romney was trying to do that last night, appealing to independent voters and Obama supporters. He was asking people to rethink whom they voted for last time around.

BP: Now, some political scientists have argued that the "bump" in the polls that conventions give candidates seems to be fading out over time. Does that suggest conventions are becoming less relevant? Or are the polls missing something?

JZ: Even a small, short bump can be worth something. It can help re-energize a campaign after a long primary. You could see by the end of the summer, Romney looked tired, he was getting beat up, and if the reaction to the Republican convention is relatively good, that's not unimportant at all. So there's more to look at than just the polls. A convention can help shape the narrative of the campaign. That's difficult to do over the summer, when people's interest in politics starts diminishing, so this is a way to recapture that attention.

There's also fundraising. People don't like to talk about it, but conventions offer a great fundraising opportunity, with all these Republicans and Democrats in one place, all these extra events. There's an unprecedented amount of money floating around.

BP: Now what about party platforms? Do those matter at all? Do they tell us much about where the parties are headed.

JZ: I’m not sure about that. There's often not much correlation between what's in the party platform and what happens with the nominee or party. I think the debate over the platform can sometimes be more important. That debate gets coverage, and that's been true going back to the 1950s and 1960s, when there were a lot of platform debates over how to talk about civil rights. These debates provide a way for reporters to think about where the party is and to discuss various issues. But in terms of what they actually say about the party, I'm not sure. John Boehner said he didn’t even read this year's GOP platform, and that's probably true. So these might continue to diminish in importance.

BP: And what about the conventions themselves? Are these going to stick around for years to come?

JZ: I could see these events becoming a lot shorter in the years ahead. That could either happen because parties see that it works fine if you have one less day of activities, or because of a drop in contributions or budget cuts. [Note: The federal government paid about $36 million in 2012 to fund the party conventions.] You could make an argument that there's no need for this much length.

And the drop in television coverage will probably also have an impact. The nature of conventions will change if the networks are only showing a little bit of each day. Parties will inevitably start focusing on just a few speeches. The conventions will become less important at showcasing new talent. This isn't really a lament for the decline of the convention, but we can probably expect some changes in how these will work in the years ahead.

Transcript has been condensed and very lightly edited for clarity.