On the second night of the Democratic National Convention in Philadelphia, delegates supporting Sen. Bernie Sanders (D-Vt.) walked out in protest, despite Sanders's calls for unity within the party. (Peter Stevenson,Adriana Usero,Dalton Bennett,David Bruns,Jayne Orenstein,Alice Li/The Washington Post)

A chief goal for Democrats' convention this week is to unify supporters of losing primary hopeful Bernie Sanders around Hillary Clinton heading into the fall campaign, a challenge that got off to a difficult start when Sanders backers repeatedly booed Clinton’s name during the day of speakers.

But beyond optics, how much of a defection threat does Clinton really face from Sanders’s primary supporters?

Much attention has been placed on the chimerical Sanders-to-Trump supporters — a group that does exist, and is fascinating, but is nonetheless quite rare. In a two-candidate matchup with Clinton, Trump garnered only 9 percent of Democratic-leaning registered voters who wanted Sanders to win the party’s nomination in combined Washington Post-ABC News polls in June and July. That’s similar to the 12 percent of non-Trump Republicans who said they would support Clinton, and it’s also lower than the share of 2008 Clinton primary supporters who professed support for McCain at this point in the campaign.

But Clinton faces a potentially more significant threat from third-party candidates who have gained ballot access in a number of states and are polling fairly well. When the poll included third-party candidates in the vote, roughly 1 in 5 Sanders Democrats (21 percent) supported either Libertarian candidate Gary Johnson or Green Party candidate Jill Stein, while slightly fewer non-Trump Republicans said they would back one of the leading third-party candidates (17 percent).

Johnson poses less of a threat to Clinton as he appears to siphon similar support from disaffected Republicans and Democrats — 10 percent of Sanders primary backers support Johnson, as do 13 percent of non-Trump Republicans.

But Stein draws disproportionately from Clinton’s ranks. Just over 1 in 10 Sanders Democrats said they support the Green Party candidate, compared with 4 percent of Republicans who supported a non-Trump candidate for the GOP nomination.

This makes sense as the Green Party’s message is far closer to the Democratic Party than the Republican, with a platform emphasizing issues such as social justice and equal opportunity, the use of sustainable energy, concern for global warming and a push for gender equality.

The saving grace for Clinton is that Stein is not very well known. Just 4 percent of Sanders-supporting Democrats mention Stein by name in an open-ended question asking which third-party candidate they would seriously consider voting for. Moreover, third-party support tends to fade late in campaigns, so Stein's and Johnson's current numbers may be a high-water mark. Stein may not even be on the ballot in some states in November, including key swing states like Virginia, North Carolina, Iowa, New Hampshire, Nevada and Pennsylvania, where the Green Party is still working to collect sufficient signatures.

We will watch to see if Stein’s campaign fundraising or name recognition picks up in the next few months — if that happens, Sanders supporters will be on defection watch.

This Post-ABC data was compiled from two national surveys. The June 20-23 poll was conducted among a random national sample including 836 registered voters reached on cellular and landline phones with an error margin of four points. The July 11-14 poll was conducted among a random national sample including 816 registered voters reached on cellular and landline phones with an error margin of four points. Combined, both of these polls included 277 Democratic and Democratic-leaning independent voters who supported Sanders in the Democratic primary, with an error margin of 6.5 points and 392 Republicans and Republican-leaning independent voters who supported a Republican candidate other than Trump in the Republican primary with an error margin of 5.5 points.

Scott Clement contributed to this report.