Former Vice President Joe Biden is poised to become the 17th presidential candidate to sign the No Fossil Fuel Money pledge, something some climate advocates say is essential to crafting policy free from the influence of polluters.

If and when he does so, Biden will join the majority of 2020 Democratic presidential candidates who have signed the pledge, which bars them from knowingly accepting more than $200 from “PACs, lobbyists, or SEC-named executives of fossil fuel companies.” That trend — and the increasing pressure for the remaining candidates to sign — highlights just how much climate change has become a major campaign issue, arguably for the first time in history.

But there is still a debate over whether the pledge itself is merely symbolic or actually has implications for policy.

“There’s always politics here,” explained Sarah Bryner, research director at the nonpartisan, nonprofit Center for Responsive Politics (CRP), which tracks money in politics.

Bryner told ThinkProgress that pledges have been a common theme this election cycle, with some candidates swearing off political action campaign (PAC) money and making other promises about their funding sources. The rejection of fossil fuel money has been among the more successful efforts, she said, but its actual impact varies.

Sens. Bernie Sanders (I-VT) and Elizabeth Warren (D-MA) are both signatories, Bryner noted, but it is unclear to what extent swearing off such donations actually impacts their funding stream. Neither Sanders nor Warren represent major energy-producing states and both are seen as solidly leftist candidates with little appeal for the fossil fuel industry.

“I can’t really speculate as to the intent of some of the candidates supporting these pledges, but certainly a lot of them believe [in this],” she said. “But it comes with the added benefit that they’re not giving up as much as some other people.”

This is in contrast to candidates like former Rep. Beto O’Rourke (D-TX), who hails from the country’s largest oil and gas producing state. O’Rourke, who already rejects PAC money, initially signed the pledge, only to backtrack, arguing that the promise precluded non-CEOs who work in the industry from donating.

After a conversation with an activist associated with the youth-led Sunrise Movement, however, he recommitted and signed the pledge — something Bryner said has real implications for his campaign. That could negatively impact his donations in an energy-heavy state like Texas, which is also true of Julián Castro, a Democratic 2020 rival who has taken the pledge and also hails from the Lone Star State.

“For someone like [O’Rourke] that’s a bigger ask… for him that’s a much bigger deal,” she said, comparing the donor implications to candidates like Sanders and Warren.

Bryner cautioned that tracking money can be hard, even for candidates, and that it is on donors to disclose who their employers are. Even so, activists argue that turning down fossil fuel money is critical to shaping climate action.

“The fact that nearly every Democratic candidate in the race has signed the pledge is a clear signal of who the Democratic party stands for,” Stephen O’Hanlon, Sunrise Movement spokesperson, said in a statement to ThinkProgress. “This is the kind of principled stand that we need from Democratic leaders if they are going to motivate young people to turn out in record numbers in 2020 to defeat [President Donald] Trump.”

Candidates are aware of that reality. Multiple polls have shown that climate change is a top concern for young people across the country, regardless of party affiliation. And younger people are set to make up a significant portion of eligible voters in 2020. Millennials are quickly surpassing Baby Boomers as the largest demographic in the country, while Generation Z will represent one out of every 10 eligible voters next year.

David Turnbull, strategic communications director with Oil Change U.S., the organization behind the fossil fuel-free pledge, expressed similar sentiments. The pledge is important “on a symbolic level,” he said, “but also a practical one.” Turnbull pointed to research that has consistently shown that donors are far more likely to get meetings with elected officials than constituents who do not donate.

“Rejecting fossil fuel money is an important starting point for candidates to show that they are serious about listening to their everyday constituents and not favor the interests of the fossil fuel executives who have the resources to max out their contributions to candidates,” he told ThinkProgress.

Experts agreed with that view. “I think in general the pledge has been remarkably successful as an activist campaign,” said Leah Stokes, an expert on environmental politics and assistant professor at U.C. Santa Barbara.

Stokes told ThinkProgress that her research has shown that fossil fuel companies have a significant influence on policy, one that typically keeps climate-related issues from even getting a vote in Congress. Even slightly mitigating the access the industry has to lawmakers, Stokes said, could have major implications for passing real climate action.

Still, she noted that the pledge itself does not preclude donations from labor unions, which have historically taken mixed stances on climate issues. Trends show that fossil fuel interests tend to fund Republicans more so than Democrats, which isn’t necessarily the case for labor.

“Labor unions have been a significant sticking point in climate legislation… that goes back a long time,” she said, noting that some unions have a major stake in sustaining fossil fuels. But, Stokes added, “labor has infinitely less money” than oil and gas interests, and some unions have been supportive of proposals like the Green New Deal — a plan to rapidly transition to a clean energy economy.

There are also other challenges inherent in efforts to create a firewall between lawmakers and special interests — namely, PACs.

Bryner, of CRP, noted that candidates only have control over their campaign committees, while PACs operate separately and can still funnel money to candidates. That’s a notable caveat for candidates like Biden. The former vice president founded the American Possibilities PAC in 2017 and its donors include former Sen. John Breaux (D-LA), who has lobbied for Shell Oil.

It is unclear whether Biden has asked American Possibilities to stop taking money from fossil fuel interests; when he said he would sign the No Fossil Fuel Money pledge, he only specified his campaign committee.

And super PACs, which operate independently of candidates, are also tricky. Those committees are barred from contributing directly to candidates, but they tend to funnel money towards attack advertisements and other efforts on behalf of individuals. The link between donors to super PACs and candidates is fuzzy at best, but as candidates aren’t in a position to reject those funds, such committees still pose an obstacle to challenging the influence of industry.

Supporters of the No Fossil Fuel Money pledge nonetheless assert that the vow still holds significant value. Turnbull argued that it is “an important potential indication” of how candidates might approach major climate action. He also noted that presidential candidates in particular set the tone for races further down the ticket, potentially influencing other lawmakers in state and local races to similarly take the pledge.

But the real test, Turnbull said, will come after the election.

“The question is,” he said, “once a candidate has removed the shackles of fossil fuel money, will they step up and put forward a plan to take the industry on directly and keep fossil fuels in the ground?”