WASHINGTON — For nearly a year, Lauren Brillante of Broomfield said she did everything she could to help elect U.S. Sen. Bernie Sanders as president. “It was like a full-time job,” said the longtime Democrat.

But on the day Sanders endorsed Hillary Clinton, Brillante said she changed her voter registration to the Green Party. And she hasn’t looked back.

“The Democratic Party isn’t representing the will of the people. They are doing it in words and not actions,” said Brillante, who now serves as state volunteer coordinator for Jill Stein, the Green Party candidate for president. “That’s the entire establishment — whether it’s Republican or Democrat.”

Brillante (pronounced Bree-AUNT) is one part of a small but growing shift among Colorado voters toward the Green and Libertarian parties. Between August 2012 and August 2016, the number of registered Libertarian voters roughly doubled — from 18,233 to 35,731 — while the Green Party rose to 11,383 voters, an increase of about 2,600.

Compared with the two major parties, these figures represent little more than a rounding error. As of Aug. 1, Republicans and Democrats together accounted for roughly 2.3 million voters in Colorado — split basically in half between them.

Individually, however, each major party claims fewer Colorado voters than the number of residents who don’t register with any party. Activists with the Green and Libertarian parties say this pool of about 1.3 million unaffiliated voters is ripe for recruits — or is at least amenable to hear their arguments during election season.

“We see that as a mandate for the Green Party. We need to step up. We need to improve our game so that people have somewhere to go,” said Andrea Merida, co-chair of the Green Party of Colorado. “What folks in Colorado are saying to these parties — loud and clear — is that they’re not representing their interests.”

The shift away from Republicans and Democrats, coupled with widespread dissatisfaction with Clinton and Donald Trump, has prompted a new sense of optimism among Green and Libertarian officials. They say this election could be the one in which they turn a toehold in U.S. politics into something more like a foothold.

“The two major parties have nominated the two most disliked candidates since ancient Rome,” said Caryn Ann Harlos, a spokeswoman for the Libertarian Party of Colorado. “I think we are looking at the death of the two-party political system as we know it.”

Interest in the Libertarian Party is “absolutely through the roof,” Harlos added. “It’s a tsunami right now.”

History suggests, however, that a major migration to U.S. third parties would require a significant change to voter behavior and the electoral system, said Sandra Davis, a political scientist at Colorado State University.

“I don’t think it’s going to be a breakthrough year for third parties,” Davis said. “It would take more of an upheaval than I think we’re seeing now.”

Part of the challenge, she said, is that third parties start at a major disadvantage. They have little money compared with Democrats and Republicans, often are blocked from participating in debates and can have trouble recruiting high-level political strategists.

Plus, she said voters who take the time to go to the polls often are aware of the long odds of third-party candidates — and often feel pressure to choose between Democrat and Republican.

“People are going to think long and hard about whether they want to vote for someone who doesn’t have a chance to win,” she said.

But Davis added that Colorado has been receptive to third parties before — and could be again. “If it were to happen, certainly Colorado would be one of those states.”

A poll released Wednesday by Quinnipiac University found that 16 percent of Colorado voters would back Libertarian candidate Gary Johnson for president and 7 percent would support Stein of the Green Party.

Both these totals are higher than what was found in Iowa and Virginia — the other two states surveyed by Quinnipiac. A separate poll by NBC News/Wall Street Journal/Marist put Johnson at 15 percent and Stein at 6 percent in Colorado.

It’s enough that Stein is planning a two-day swing through Colorado at the end of August. She’s looking to improve on the 2012 performance of third-party presidential candidates in Colorado. That year Stein won 7,508 votes, and Johnson won 35,545.

Historically, there are signs Colorado is open-minded toward third-party bids.

The state is the birthplace of the Libertarian Party, and during the 1992 presidential contest about 23 percent of Colorado voters backed independent Ross Perot for president. Perot won two Colorado counties: Moffat and San Juan.

Even now there is a smattering of third-party officials in the state. Beau Woodcock, the mayor of Milliken, is Libertarian, and Art Goodtimes of the Green Party serves as a commissioner in San Miguel County.

“It really comes down to local empowerment, freedom and citizen engagement,” Woodcock said. “Unfortunately, both the Republicans and Democrats have strayed from this in favor of big centralized government, which has proven to be ineffective in both preserving freedom and being able to address the needs of the citizens and communities.”

Nearly 30 Coloradans are running as Libertarian this year, and a handful have taken up the Green banner as well.

Among them: Libertarian Lily Tang Williams and Green Arn Menconi — both of whom are trying to play spoiler in a Senate race where Republican Darryl Glenn also is looking to unseat Democrat Michael Bennet.

“People have been fed up for a while. This is just a breaking point,” said Williams, who also noted the unpopularity of Clinton and Trump. “That’s why third-party candidates this year are shining.”

Said Menconi: “Is this a David and Goliath fight? Of course it is. But things can change, and they can change rapidly. And David won.”

Even if Green and Libertarian candidates are unsuccessful in November, the movements have drawn the attention of Democratic and Republican strategists.

“Both parties should be examining themselves to ensure their tent is ultimately big enough to bring them in,” said Rick Ridder, a Democratic consultant.

Added Owen Loftus, a Republican operative: “This year has to the potential to be different, and the reason is Donald Trump.” Loftus warned the GOP could see future defections to third parties unless Republican leaders steered the party’s message away from Trump’s negative rhetoric toward Muslims and immigrants.

“Republicans have to be that party that brings hope and can make the case for why we need conservative leadership,” Loftus said.

Meanwhile, one of the early activists in the Libertarian movement said he sees a different lesson ahead.

“The real test will be how people perceive the party going into the 2018 election,” said D. Frank Robinson, 73, a longtime activist with the Libertarian Party of Oklahoma.

He said Green and Libertarian parties should concentrate more on the number of new registered voters and the fate of down-ballot races — rather than the results of the presidential contest. There, he said, are the seeds of long-term change.

“To this day, I have not been able to dissuade myself that focusing on the Congress is the better strategy than focusing on the presidency,” he said.

If those numbers show significant improvement, Robinson added, there is a “very high potential (for a) new beginning.”