Alexandria Ocasio-Cortez, the Democratic socialist congressional candidate who has spent weeks campaigning with fellow socialist Sen. Bernie Sanders, on Thursday refused to endorse his prospective candidacy in the 2020 presidential race.

Ocasio-Cortez, 29, worked as an organizer for Sanders' failed presidential campaign in 2016 before winning in a surprise upset over longtime establishment Rep. Joe Crowley, D-N.Y., in the state's 14th Congressional District primary in May.

"She’ll see what the field looks like," Corbin Trent, Ocasio-Cortez’s communications director, said in an interview with Politico. "She’s focused on 2018, [Bernie’s] focused on 2018. We’re all focused on 2018."

For his part, the 77-year-old Sanders did not endorse Ocasio-Cortez's run against Crowley. And in August, former President Barack Obama, in turn, initially declined to endorse Ocasio-Cortez even after her stunning primary win, underscoring the challenges facing progressives campaigning to the left of the Democratic Party establishment in hopes of taking their views mainstream. (Obama endorsed her earlier this month.)

Despite the back-and-forth snubs, Sanders and Ocasio-Cortez have nonetheless teamed up in the past several months to tout socialist and far-left candidates throughout the country during the midterm season, to varying degrees of success.

"She’ll see what the field looks like." — Alexandria Ocasio-Cortez’s comms director Corbin Trent

For example, socialist-backed candidates have prevailed in Pennsylvania, Kansas, and Washington. But Abdul El-Sayed, an avowed populist who was vying to be Michigan's first Muslim governor, finished nearly 20 percentage points behind establishment Democrat Gretchen Whitmer in the state's August primary, despite the backing of both high-profile socialists.

Also in August, Fayrouz Saad, who was angling to become the first Muslim woman elected to Congress, finished in a distant fourth place in Michigan's 11th Congressional District primary, despite support on the stump from Ocasio-Cortez.

Similarly, in Missouri, Rep. Lacy Clay soundly defeated far-left progressive Cori Bush. And in Hawaii in August, a fiscally conservative Democrat, Ed Case, took down 29-year-old Kaniela Ing, a democratic socialist who had Ocasio-Cortez's endorsement.

For Sanders, who has kept the door open on running again for the White House in 2020, the defeats were a continuation of his recent losing streak. Over the past two years, several candidates he has backed in several important races -- including in gubernatorial primaries in Virginia and Ohio, and in several House races in Iowa and New Jersey -- have come up short.

Some former Sanders staffers, meanwhile, have said in interviews they hope to find another torchbearer in 2020 for the Vermont senator's beliefs.

“All the former staff I’ve talked to agree Bernie should focus on making sure the nominee is someone who continues what he started,” Keegan Goudiss, Sanders’ former digital advertising director, told Politico. “It’s a sure thing that he will be able to influence 2020 from the outside. But if he decides to run, I doubt many former staffers will return unless directly asked to. Either way, he misses capturing energy if he doesn't decide soon.”

The magazine quoted several other former Sanders staffers with a similar perspective on Sanders' future, although some did say they would support him running again for the presidency.

One of those people, Sanders spokesman Jeff Weavers, who also ran his 2016 campaign, wrote a book published earlier this year titled “How Bernie Won."

“Bernie Sanders is an extremely energetic and vigorous person, and has more energy, I would say, than people half his age,” Weavers said in an interview, dismissing criticisms that Sanders would be too old to compete in 2020.

His book concludes: “Run, Bernie, Run!”