(CNN) Before Sen. Bernie Sanders emerged as the left's alternative to Hillary Clinton in 2015, many of the same activists who eventually helped lift his primary campaign had a different progressive hero in mind: Sen. Elizabeth Warren.

At least two major efforts to draft Warren into the race folded late that spring when it became apparent the Massachusetts Democrat would sit out the race. In the aftermath, the founders of "Ready for Warren" launched "People for Bernie," a grassroots group that took off along with Sanders and is expected to be an active organizing force again in 2020.

Four years later, Warren and Sanders, who kicked off his second presidential campaign on Tuesday , begin the Democratic primary not as insurgents or slingshot-yielding giant-slayers but among the early favorites in a historically deep field of contenders vying to unseat President Donald Trump. For progressives, there is an element of the bittersweet in what's to come: two candidates they trust and credit for driving the Democratic Party's charge to the left are poised now to compete for the same votes — and, if they advance far enough in the contest, potentially divide their largely shared base of support.

"People for Bernie" co-founder Winnie Wong helped seed the group, which now features a Facebook page with nearly 1.5 million followers, by appealing to the names on an email list initially built during that 2015 effort to entice Warren.

"We decided (in 2014) that Elizabeth Warren was the best, most authentic messenger to convey to the rest of the country the message about income inequality," Wong recalled in an interview. But when it became apparent in the spring of 2015 that Warren would not run, "we decided to pivot," she said, and support Sanders.

The Vermont senator was, at that point, still a relative unknown in mainstream American politics. But where most pundits and many reporters saw a dead end, progressive activists with an eye on his ability to go viral recognized an unprecedented opportunity.

"Here we had a man who we knew had almost no name recognition and who openly identified as an independent socialist, who had given numerous speeches on the floor that, to the left, were heralded as acts of great political bravery," Wong said. "So for us on the left, he was really regarded as a real icon. And he still is."

The desire of so many Democrats to finally elect the first woman president, along with differences in age (Warren is 69; Sanders is 77), could ultimately help decide how the progressive vote breaks. But there are also deeply rooted and sincere ideological tensions that separate the friendly Senate colleagues.

Sanders, a democratic socialist, and Warren, a capitalist to her "bones," both describe the American economy and political system as "rigged" to benefit rich corporate interests. But they have diverging views on the roots of the problem and the depths of reform needed to break its grip.

Warren believes government-enforced restrictions, like those outlined in her "Accountable Capitalism Act," could unwind a decades-long trend of consolidation at what she describes as the "tippy top" of the economic ladder. Sanders argues that the rot goes much deeper and that a fundamental reordering of the economy -- most purely viewed in his long crusade for single-payer health care, which would cut the private sector out of one of America's largest industries -- is required to empower a "political revolution."

They diverge -- narrowly -- on health care. Warren is a cosponsor of Sanders' Medicare for all legislation, but she has not ruled out a role for private insurers going forward.

"There are multiple bills on the floor in the United States Senate. I've signed onto Medicare for all. I've signed on to another one that gives an option for buying in to Medicaid. There are different ways we can get there," Warren said in a recent interview with Bloomberg . "But the key has to be always keep the center of the bull's-eye in mind. And that is affordable health care for every American."

Sanders, who, like Warren, has signed on to more moderate bills, isn't opposed to incremental steps. But his endgame is simple: to remove big business, and with it the profit incentive, from any meaningful role in Americans' health care.

"The fight for Medicare for all will be opposed by all of the special interests -- drug companies, insurance companies, Wall Street -- who make billions from our dysfunctional health care system," Sanders tweeted two weeks ago, adding in a refrain he used in the email announcing his run: "They may have the money. But we have the people. We are going to win this fight."

Whether those divisions, and what they imply about how the candidates would govern if elected, register with a sizable share of the primary vote is likely to remain a mystery for the next year, as the primary's progressive bloc -- which could still be joined by Sens. Jeff Merkley and Sherrod Brown -- shakes itself out.

Activists with outside attachments to the two campaigns believe that the presence of Sanders, Warren and other leading progressive in the race will, at least in these early stages of the primary, boost the cause and to make it politically untenable for Democrats to revert to more business-friendly ways. In the last few weeks, the senators have released complementary proposals -- Warren's tax on Americans with assets of $50 million or more and Sanders' plan to expand the estate tax on the wealthiest 0.2% of households -- both aimed at closing the country's staggering wealth gap.

"There are some in the intellectual class who draw distinctions between socialism versus capitalism and making markets work better for consumers and small business, but honestly I just don't think most voters are going to think about labels as much as the core impact on their lives," said Adam Green, a co-founder of the Progressive Change Campaign Committee, which has endorsed Warren in 2020 and was one of her earliest grassroots political backers. "And I think it's fair to say that Elizabeth and Bernie are both generally going in the same direction and have the same height of ambition there."

At Warren's early events on the campaign trail, voters who cheered her attacks on corporate giants and the political ruling class also spoke proudly of backing Sanders in 2016. Asked about how they might break in 2020, many winced or seemed pained by indecision.

In the parking lot outside a January rally in Council Bluffs, Iowa, 27-year-old Liz Higley purchased and pulled on a "Nevertheless, she persisted" T-shirt. She too had wanted Warren to run in 2016 but happily backed Sanders in the end. Higley said the prospect of choosing between them now made her uneasy.

"I'm really partial to Bernie because I wanted to vote for him (for president) before, but I would like to see a woman take it," she said of the Democratic nomination. "So I probably would lean toward her. But I wish they would just run together and make everyone happy."

Natalie Wertz, a 29-year-old from Omaha, Nebraska, who is supportive of both Warren and Sanders, said if confronted with a primary day choice between the two senators, she would lean toward the Massachusetts Democrat.

"Not to be ageist," Wertz said, "but she's a lot younger."