In a typical presidential nomination contest, several candidates compete for the prize, and the losers bow out and endorse the winner. This isn't always a clean process — bad blood and important policy differences may remain — but conventions tend to present a united front, with the various candidates having made peace. But we're seeing some evidence from this week's Democratic convention that things are a bit different from usual in that party.

The yardstick that I and many other political observers have been using is the example of Hillary Clinton's 2008 presidential bid. The contest between Clinton and Barack Obama was considerably closer than this year's Clinton/Sanders contest, with Clinton having enjoyed an early advantage in national public opinion polls and endorsements. The 2008 contest was also far more negative than this year's was, with the candidates frequently questioning each other's likability and fitness for office.

It was obvious by spring of 2008 that that math was against Clinton. By the beginning of June, when the last of the primaries were held, she had come up short, and Barack Obama was the presumptive nominee.

But it was still nearly three months until the convention, and many of her ardent supporters were not prepared to give up. They were angry with Obama and his team. They felt he hadn't treated Clinton respectfully, that he'd won unfairly due to a number of disputed delegate counts in Michigan and elsewhere, and that Clinton was the true choice of the party's voters. Several of them created an organization called PUMA (officially, People United Means Action; unofficially, Party Unity My Ass) and threatened to vote for John McCain in the fall.

Judging from public opinion polls and my own observations on the convention floor, Clinton's most ardent backers overwhelmingly followed her enthusiastic endorsement of Obama. PUMA's threats never materialized. No Clinton delegates led a walkout during the convention or tried to shout over speakers.

The tone is definitely different this week. While most of Sanders's backers have indicated their support for Clinton, a rather vocal minority has refused to do so. Some of them are audibly chanting over podium speakers, and some led a walkout after the roll call vote in which Sanders moved to nominate Clinton by acclamation. Several of those who walked out even marched with Jill Stein, the nominee of another party.

Why is this going on? What makes the losing candidate's supporters in 2016 so much more resistant to supporting the winner than the losing candidate's supporters in 2008?

35 percent of Hillary Clinton's delegates in 2008 had attended a previous Democratic National Convention, while only 21 percent of Barack Obama's delegates had. The losing candidate's supporters actually had more experience within the party.

Matt Yglesias nails the answer when he notes that a lot of Sanders's delegates don't really come from the Democratic Party. Some are brand new to politics. Many, Yglesias notes, come from the world of political activism. Their loyalty is not necessarily to the party or even to Sen. Sanders, but to an issue or an agenda. They will support a candidate or a party only to the extent that it is a way to advance the agenda.

In 2008, I was part of a research team that surveyed hundreds of delegates at both major party conventions. We found that 35 percent of Hillary Clinton's delegates that year had attended a previous Democratic National Convention, while only 21 percent of Barack Obama's delegates had. The losing candidate's supporters actually had more experience within the party. We haven't seen any hard numbers yet about this year's delegates, but what evidence we have suggests that most of Sanders's supporters have very little history within the party.

All of this is consistent with what we saw during the invisible primary before any of the actual voting started this year. The Democratic establishment indicated in basically every way it knew how that Clinton was its preferred candidate. Democratic elected officials overwhelmingly endorsed her over Sanders. Most potential presidential candidates with any real experience within the party simply stayed out of the race.

It was the same with those who sought to serve as delegates. Longstanding party people either signed up for Clinton or didn't sign up at all. The Sanders people came from elsewhere.

It will be interesting to see where Sanders's delegates end up over the next few years. Some will likely stay with the Democratic Party and continue to press for many of the issues Sanders championed in his campaign. A good many, however, will return to the activist community and press for change outside the party system. From many of their perspectives, they never left it.