Bernie Sanders has urged his supporters to back Hillary Clinton—a point he reiterated in an op-ed at the Los Angeles Times last week.

But a string of recent polls shows that about one-third of Sanders backers aren't willing to do that, as political website FiveThirtyEight points out.

Harry Enten writes Monday that since the Democratic National Convention, Clinton has seen a boost in support from Sanders supporters. Looking at the average of four post-convention polls—CNN, Fox News, Marist, and YouGov—78 percent of Sanders backers said they'd vote for Clinton compared to nine percent for Donald Trump when presented with a two-way match-up. That's up from about half of Sanders supporters ahead of the conventions, he writes.

But when given the option of third party candidates, the number drops, with the four polls showing an average of 63 percent of Sanders supporters saying they'd back Clinton.

Enten writes: "the sizable portion of Sanders supporters defecting from Clinton when given other options could still be a problem for the Clinton campaign if the election tightens." That echoes Politico's suggestion last week that there could be a potential "third-party headache" for the former secretary of state.

Still, Enten adds, Clinton "may not need their help," noting that "she leads nationally by 6 or 7 percentage points at the moment."

Speaking Thursday in Portland, Maine, the Republican presidential nominee said his ticket would score "a lot of Bernie Sanders votes because of trade." But as Newsday reported, Sanders' supporters are not flocking to the real estate mogul.

In its reporting on Sunday, the Long Island paper cited the CNN/ORC poll from last week which showed that just six percent of Sanders backers would vote for Trump's ticket.

The Green Party's Jill Stein, meanwhile, is hoping to woo Sanders' supporters. She told her party's convention this weekend that she was excited "to be running in alliance with the Bernie Sanders movement that lives on outside the Democratic Party," and called for "a new way forward based on democracy, justice, and human rights. And that won't come from corporate political parties funded by predatory banks, war profiteers, and fossil fuel giants."

Even though she's polling far better than she did during her 2012 run, "[t]here are some big hurdles for Stein before she can be a real factor on the ballot this fall," NPR reported. "She needs to get to 15 percent to just make the debate stage, and right now she's polling at about 5 percent."

An ABC News/Washington Post poll released Sunday presenting a two-way match-up shows Clinton with an eight-point lead—50-42 percent—over Trump among registered voters. When presented with a four-way match-up to include Libertarian Gary Johnson and Stein, Clinton still has an eight-point lead over Trump. In that scenario, Clinton got 45 percent, Trump 37 percent, Johnson 8 percent, and Stein 4 percent.