For many liberals, the energy around Bernie Sanders' upstart campaign for president is a sign that long-awaited change is about to come to the Democratic Party.

The self-proclaimed socialist has undeniably tapped into the frustrations of many on the far left who feel ignored by the centrist positions of the last two Democratic administrations. But while he's inspiring Democrats by the thousands and already had a demonstrable effect on Hillary Clinton's campaign – pushing her to embrace a debt-free college tuition plan and a raising of the minimum wage – his candidacy isn't likely to translate into a movement that could carry a wave of far-left candidates into office.

In other words, a progressive answer to the tea party probably isn't coming.

The record-breaking crowds at Sanders' campaign rallies reveal a real anxiety within part of the Democratic Party that the establishment is too willing to compromise toward the center. But it's not as if the Sanders bid is the first time progressives have rallied around a candidate, only to see their efforts fail to make a major impact on the party: Sanders' massive events have drawn comparisons to other candidacies that later fizzled, from Eugene McCarthy in 1968 to Howard Dean in 2004.

So why do progressives struggle to gain a lasting foothold?

Not only do challengers often lack the kind of platform and organization that can heavily advantage incumbents or so-called "establishment" candidates, but progressives face a panoply of other factors that keep many from running.

To start, liberals may be wary of challenging a fellow Democrat, hesitant to unseat an incumbent only to lose the general election to a Republican. For example, although many races are still shaping up, only a few incumbent House Democrats are facing contested primaries, and none of the seven Senate Democrats running for re-election has attracted a challenger. Democrats are facing off in primary races for Senate seats in California, Illinois, Pennsylvania, Maryland and Florida, but only in Maryland and Florida, two open seats, have candidates attempted to align themselves with progressives in bids to outflank establishment choices on the left.

That's in stark contrast to the modern GOP, which starting with the birth of the tea party, demonstrated a readiness to take down their own from the right.

"Tea party activists did not focus their attention exclusively on any one candidate; they would push candidates to the right with the understanding that their point was to make the whole Republican Party more conservative," says Vanessa Williamson, a fellow at the Brookings Institution and the author of "The Tea Party and the Remaking of Republican Conservatism."

"There was a general willingness to lose," she says of how the tea party was able to gain a foothold in the GOP. "The occasional losing candidate doesn't matter if what you get is a party that's in lockstep on the issues you care about."

It's also possible that part of the difference in behavior is nature, not nurture: A variety of studies in psychology, brain science and behavioral science in recent years have contributed to a growing consensus that liberals and conservatives are wired differently.

"There is by now evidence from a variety of laboratories around the world using a variety of methodological techniques leading to the virtually inescapable conclusion that the cognitive-motivational styles of leftists and rightists are quite different," wrote John Jost, a New York University psychologist in response to a metastudy released in the journal Behavioral and Brain Sciences in 2014. "This research consistently finds that conservatism is positively associated with heightened epistemic concerns for order, structure, closure, certainty, consistency, simplicity and familiarity, as well as existential concerns such as perceptions of danger, sensitivity to threat, and death anxiety."

Those factors, called "negativity bias," are one explanation for why conservatives are more likely to form and adhere to consensus, which allowed them to harness the energy behind the tea party and turn it into substantial electoral victories in 2010 and 2014.

Joshua Grossman, the president of Progressive Kick, a super PAC aimed at electing progressives to Congress and state legislatures who describes himself as "envious" of the tea party's success, says he sees this distinction in practice.

"It's something Republicans have much more stomach for," he says. "I love how they punish their own villains, and I only wish we punished our villains the same way."

On the front lines of the effort to urge more progressives to run for office is Elizabeth Warren, the Massachusetts senator who has shown a knack for hitting on unabashedly progressive themes with fiery speeches that do well on YouTube and speak to the left's discontent with establishment politics. Warren's brash advocacy for the Consumer Financial Protection Bureau and her devastating takedowns of big banks made her a progressive darling, and the so-called "Warren wing" of the party was quick to embrace Sanders – once Warren convinced them she would not run for president.

At a training event put on by the Progressive Change Campaign Committee earlier this month, Warren spoke to 200 potential candidates and activists, hoping to urge them to run for office and drive the political conversation.

"You are the progressive bench, and we need a bench," Warren told the crowd at the Washington Court Hotel. "This is about building campaigns and winning office, but this is also about building a movement."

Implicit in her comments was an acknowledgement that Democrats, progressives in particular, have lagged behind conservatives in recruiting candidates. And while the excitement of her fans – and Sanders' fans – is palpable, it has yet to translate into challenges.

"I would like to be wrong, but I don't see a flood of progressive candidates infusing congressional elections," Grossman says.

Even when they show interest in running, progressives' resistance to taking campaign contributions from big corporate interests, in favor of grassroots donations, can make sustaining energy around a bid difficult. Reliance on small donors can put their campaigns at a significant financial disadvantage, especially in a post-Citizens United world, where independent groups can spend unlimited money on elections.

"Having to raise so much money to be competitive in a campaign, not only would that take away from the time to do what they thought was really important, like talking to people, but there was also an 'ick' factor," says Shauna Shames, a professor of political science at Rutgers University, who has studied the reasons why people choose not to run for office.

That can prove a problem for a campaign's ability to raise funds; their core supporters, who on average are younger, poorer and more likely to be a minority simply can't match the right.

"The work that's been done to harness the anti-Obama energy on the right has been pretty effective because the activists involved are older and on average wealthier and have more spare time," Williamson says. "They can be involved in a campaign for the long haul, in a way that young people often can't."

Even if Sanders does inspire more progressives to run, the effect might not be felt for two years or four, or more likely in 2022, after the next Census and redistricting.

"It's possible that actually what Bernie would want to come out of the candidacy – that he doesn't really want to be president, he would rather inspire a new generation of liberal candidates," Shames says.

One candidate for Congress in New Jersey, 24-year-old Sanders supporter Alex Law, acknowledges that those who are energized by Sanders' campaign are probably already too late for 2016.

"If you want to run in the 2016 cycle, you need to have already started," says Law, who is challenging freshman Rep. Donald Norcross, D-N.J., and who began his campaign six months ago. Law argues that the incumbent is just an extension of a political machine that rewards the powerful and disregards the vulnerable.

He also acknowledges the usual obstacles to entry are somewhat lower in his safe Democrat district, where only a fraction of registered voters will show up on primary day, dramatically lowering the amount of money he'll need to win.

Law planned to support Clinton until Sanders jumped in, and says he would endorse her in the general election if she beats Sanders in the primary. He says he's pleased that Sanders' seems to be pushing Clinton to stake out definitive, progressive positions, but in the meantime, Law is hoping Sanders' campaign will lift him to victory. On July 29, when Sanders held a live-streamed national rally with thousands of watch parties around the country, Law says he was able to speak with some 400 people who attended a local event.

Still, he says he expects the biggest difference to come in future elections.

"People from all over the country are writing to me, saying they're interested in running for office," Law says, "but I think it will take until the next cycle until we see a lot more progressive candidates."