Vanessa Tijerina was not politically active until a couple years ago. Then she started looking closely at problems in the healthcare system, feeling that mainstream politicians were not delivering solutions – and discovered Bernie Sanders.

Last September, at short notice, she drove three hours to San Antonio, Texas, from her home near the border with Mexico so she could join other Sanders supporters in a protest against a Democratic congressman who had criticised the Vermont senator. In January she marched through Manhattan for Sanders, inspired by his idealistic and unorthodox message.

And then the 74-year-old lost the Democratic presidential primary to Hillary Clinton, and backed her for the White House last month at the national convention. That was where Tijerina and Sanders diverged.

“A lot of us waited with bated breath, wondering: what’s he going to do?” she said. “Because depending on what he did, that’s where the movement was either going to go or not go. He decided to stay there, and the movement can’t stay there. It cannot stay in a centrist environment. The movement is antagonising that environment so we can’t stay there.”

Tijerina and others were calling for a new phase of a Sanders-style political revolution on Thursday, as the Green party kicked off its presidential nominating convention in the improbable location of Houston, Texas – Big Oil’s back yard. (The convention’s slogan is: “Houston, We Have a Solution”.)

“To me the Green party was the only other option,” Tijerina said in a conference room at the University of Houston. “There’s just no way that anything centre or right of centre was going to get America where it needed to be.”

Vanessa Tijerina: ‘There’s just no way that anything centre or right of centre was going to get America where it needed to be.’ Photograph: Tom Dart

The 38-year-old nurse is standing as a Green party candidate for a Texas congressional district this November.

“When I found out about Jill Stein, which was literally a month after I found out about Bernie – depending on who you ask, she is perhaps more progressive than Bernie,” Tijerina said. “She is perhaps more aggressive politically than Bernie and she is perhaps, some would say, less afraid, or less intimidated – or whatever it is, she has what Bernie has and perhaps more.”

Stein is set to be confirmed as the Green party’s nominee on Saturday and is openly courting the seemingly large number of Sanders supporters who are reluctant or refusing to heed his call to support Clinton. In a Real Clear Politics average of recent polls, Stein is at 3.9%, behind the Libertarian party’s Gary Johnson, at 8%; Donald Trump is at 36.9% and Clinton at 43.5%.

Hardly numbers to threaten the duopoly, but enough to potentially complicate close races in swing states in November and indicative of significant unhappiness with the Democratic and Republican candidates. (In 2012, Johnson received only 0.99% and Stein 0.36% of the popular vote.) Enough also to awaken some deep-seated Democratic angst rooted in the trauma of 2000, when some blamed Green party candidate Ralph Nader’s presence in Florida for costing Al Gore the general election and sending George W Bush to the White House.

Adding to the sense of the Green party as an irritant for Democrats, WikiLeaks founder Julian Assange may address the convention via video link on Saturday, about two weeks after carefully timed leaked emails showed that Democratic National Committee officials seemingly plotted to undermine Sanders’ campaign, causing a storm and adding to his loyalists’ antagonism towards Clinton and the party.

Stein’s choice this week of the human rights activist Ajamu Baraka as her running mate might complicate the outreach. In a blogpost last September on the left’s response (or lack of it) to Saudi Arabia’s bombing of Yemen, Baraka wrote: “Sanders supporters have not only fallen into the ideological trap of a form of narrow ‘left’ nativism, but also the white supremacist ethical contradiction that reinforces racist cynicism in which some lives are disposable for the greater good of the west.

“And as much as the ‘Sandernistas’ attempt to disarticulate Sanders ‘progressive’ domestic policies from his documented support for empire … it should be obvious that his campaign is an ideological prop – albeit from a center/left position – of the logic and interests of the capitalist-imperialist settler state.”

A request for comment made through Stein’s campaign was not returned. But it would likely take much more than some debatable blog articles to persuade many of the convention-goers in Houston to vote for an establishment figure they see as emblematic of a dishonest, warmongering status quo that is in thrall to big corporations and unwilling or unable to solve systemic inequalities.

“Voting for the oligarchy is not how you get rid of the oligarchy,” said Carlos Martinez, 40, an activist from Texas who creates social media content. “I in good conscience cannot vote for somebody that supports interventionist wars and supports what Hillary Clinton supports, and I will vote for Jill Stein.”

Sanders, some Clinton supporters – and Trump himself – have argued that casting a ballot for the Green party is tantamount to helping the Republican candidate. Stein and her supporters, of course, reject that idea.

“I really can’t tell who or what Donald Trump is,” Martinez said, “but that doesn’t mean that I’m going to be fear-baited into supporting Hillary Clinton and I don’t think that the majority of Sanders supporters are scared to vote their conscience and they’re going to be fear-mongered into pulling the lever for somebody that’s against their own interests.”

Pam Ellis on her dislike of Hillary Clinton: ‘Nothing specific or anything, I just feel like it’s more big money influence in politics.’ Photograph: Tom Dart

Wearing a red “Feel the Bern” T-shirt, Pam Ellis attended a boisterous evening event designed to woo the “Bernie or Bust” crowd that drew a couple of hundred people.

“I have a conflict because I like Bernie; I never really cared that much for Hillary. Nothing specific or anything, I just feel like it’s more big money influence in politics,” the 57-year-old nurse said. Her top issues were access to education and healthcare, subjects that were at the heart of Sanders’ electoral pitch – focused, like Stein’s, on social justice.

“I think he furthered the Green party platform whether he intended to or not, brought it to the forefront and caused me to do a lot of research on the Green party, and that’s why I’m here now,” she said.

Ellis also dismissed the notion that voters should think tactically. “What good is it to live in the United States where you have the right to vote and the freedom to vote for who you want to, if you’re going to be scared into voting?” she said.

“I’m tired of the lesser of the evils. I’m just sick of that. That’s not what my father and his brothers went and fought in world war two for, so we could be scared into voting for one of two candidates. This is a huge country. We should have debates with the Libertarian party and the Green party. We need to hear from everybody.”