A recurring series of Insider polls found that supporters of Sen. Bernie Sanders are now more likely to support Sen. Elizabeth Warren over former Vice President Joe Biden as the Democratic nominee, a significant shift as the primary race heats up.

The data indicates a serious shift in the loyalties of Sanders supporters.

Shortly after Sanders entered the race in February, around 59% of the Vermont's senator's self-identified supporters in the poll indicated Biden would be a satisfactory nominee.

Then the tide started to change in early June when Sanders supporters began expressing more favorable views of Warren, a sign they started connecting with her own fiery message of economic populism.

That trend accelerated after the first Democratic primary debate later that month, and Biden's previous satisfaction figures never recovered among Sanders supporters.

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A recurring series of Insider polls found that supporters of Sen. Bernie Sanders are now more likely to support Sen. Elizabeth Warren over former Vice President Joe Biden as the Democratic nominee, a significant shift as the primary race heats up.

The data indicates a serious shift in the loyalties of Sanders supporters and underscores Warren's steady rise in the polls for the party's nomination.

Insider has since December been polling how different candidates are perceived — as satisfactory or unsatisfactory nominees — among self-identified primary voters. As of September 25, around 65% of Sanders supporters expressed satisfaction with Warren as the Democratic nominee. That's compared to 52% who said they'd be satisfied with Biden.

Shortly after Sanders entered the race in late February, around 59% of Sanders supporters indicated they had favorable views of Biden and would support him to face off against President Donald Trump in the general election. It was a trend that held consistently through the spring as Warren struggled to break out of the crowded primary field. At one point, Biden had a 23-point favorability lead over Warren among Sanders supporters on April 5 in the Insider poll, 60% compared to 37%.

This suggests Sanders supporters may not have been as ideological as previously thought, and are eager to support someone who musters the best chances to defeat Trump in the 2020 election.

The tide started to change in early June when Sanders supporters began expressing more favorable views of Warren, a sign they started connecting with her own fiery message of economic populism. That trend accelerated after the first Democratic primary debate later that month, and Biden's previous favorability figures never recovered among Sanders supporters.

Furthermore this is interesting because as far as overall levels of support go, Sanders has remained steady since his entrance: about half the people who know of him would be satisfied with him as nominee, and about a fifth would be unsatisfied with him. Those numbers have not shifted significantly over the course of the cycle.

Read more: Elizabeth Warren is the only 2020 presidential candidate who has locked down way more supporters than she had when she started running

The data is remarkable.

Sanders has long positioned himself as a democratic socialist, railing against wealth inequality, pressing for universal healthcare and combating climate change. It's the polar opposite of Biden, who has campaigned as a moderate.

Still, early on in the race the two were the only ones with a history of mounting campaigns at a presidential level, a familiarity that may have fueled some to view the ideologically competing candidates as satisfactory regardless of their personal belief.

Warren and Biden also represent competing visions for the nation's political future.

The Massachusetts senator has embraced a progressive message around economic populism, having rolled out a stream of plans calling for "big, systemic change" that would reshape the American economy. Biden, however, has centered his appeal around a centrist message of restoring the Obama years. He's repeatedly blasted Trump on the campaign trail.

Insider asked over 1,100 respondents per poll from December 2018 to September 2019 about their preferences. All of our respondent data can be downloaded and more details on how we poll and what we ask can be found here.

Read more: How the INSIDER 2020 Democratic primary tracker works

Sanders and Warren are similar politically, but there's a key difference

Throughout the campaign, Warren and Sanders have fought to win the hearts and minds of the Democratic Party's progressive wing. Last week, Sanders co-opted the wealth tax that's helped Warren electrify her crowds at campaign rallies with chants of "two cents!" highlighting how many cents on the dollar those with fortunes of $50 million and over would have to pay up.

But Warren has repeatedly described herself as a "capitalist to my bones," seeking to carve out space in the progressive lane, but not going as far toward revolutionary as Sanders.

Warren has since emerged as Sanders' strongest rival, as she competes for the same pool of progressive voters forming the bedrock of his base. Should her rise continue unimpeded, the odds steadily increase Warren could be the party's next nominee for president.