Democratic presidential contender Julian Castro launched his campaign by pledging support for 'Medicare for All,' free universal preschool, a large public investment in renewable energy and two years of free college for all Americans. That wasn't enough for some of his party's most liberal members. Critics on social media quickly knocked Castro's plan to provide only two years of free higher education — instead of four — as 'half measures,' ''scraps' and 'corporate Dem doublespeak.' Aware of the backlash, the former Obama administration Cabinet member clarified his position in an interview days later. 'At least the first two years of college or university or apprenticeship program should be tuition free — and preferably four years,' Castro told The Associated Press. 'We're going to work toward that.' Welcome to the 2020 presidential primary. Almost no policy is too liberal for Democrats fighting to win over their party's base, which is demanding a presidential nominee dedicated to pursuing bold action on America's most pressing challenges. Among two dozen possible candidates, virtually all have embraced universal health care in one form or another. Some have rallied behind free college, job guarantee programs, a $15 minimum wage and abolishing — or at least reconstituting — the federal agency that enforces immigration laws. While few have outlined detailed proposals to fund their priorities, most would generate new revenue by taxing the rich. The leftward lurch on top policies carries risks. President Donald Trump and his Republican allies are betting that voters will ultimately reject the Democratic proposals as extreme. Some GOP leaders cast lesser plans as socialism during the Obama era. Republican critics are joined by a handful of moderate Democrats, who fear that promises by well-intentioned presidential prospects may create unrealistic expectations with their party's most passionate voters. Billionaire businessman Michael Bloomberg, a former Republican mayor of New York now considering a Democratic presidential bid, recently opined that primary voters might be receptive to a more moderate approach. 'Most Democrats want a middle-of-the-road strategy,' Bloomberg said on ABC's 'The View.' He added: 'If you go off on trying to push for something that has no chance of getting done, that we couldn't possibly pay for, that just takes away from where you can really make progress in helping people that need help today.' So far, at least, very few presidential prospects are heeding such warnings. In the 2016 campaign, Vermont Sen. Bernie Sanders, a self-described democratic socialist, was the only presidential contender to support 'Medicare for All,' a proposal that would essentially provide free health care coverage to all Americans. This year, it's hard to find anyone in opposition. That's even after one recent study predicted the plan would cost taxpayers more than $32 trillion. Proponents argue that those same taxpayers would save the trillions they currently spend out-of-pocket for their health care. Lesser-known policies have emerged heading into 2020 as well. New Jersey Sen. Cory Booker, who is expected to launch his presidential campaign soon, has sponsored legislation to create a federal jobs guarantee program in several communities across America. The pilot program, which is co-sponsored by fellow 2020ers like New York Sen. Kirsten Gillibrand, Massachusetts Sen. Elizabeth Warren, California Sen. Kamala Harris and Oregon Sen. Jeff Merkley, could ultimately transform the U.S. labor market by providing well-paid government employment with benefits for anyone who wants it. Critics decry the plan as a step toward socialism. 'Big challenges demand big solutions,' Booker told the AP. 'Both Martin Luther King Jr. and President Franklin Roosevelt believed that every American had the right to a job, and that right has only become more important in this age of increasing income inequality, labor market concentration and continued employment discrimination.' Billionaire activist Tom Steyer supports much of the liberal movement's new priorities — including Trump's impeachment — but says the federal jobs guarantee 'doesn't make sense' given the nation's low employment rate. 'I want the private sector to produce jobs people can live on,' he said in an interview. 'A guarantee of government jobs doesn't make sense.' Yet Steyer insists that most of his party's policy priorities — universal health care and free college, among them — are anything but radical. 'The Republicans are an extremist far-right, radical party. When you say we need to moderate to their position, there's nothing moderate or pragmatic about their position,' said Steyer, who recently backed away from a presidential run, although he's expected to spend tens of millions of dollars to shape the 2020 debate. Free college is quickly emerging as a litmus test for Democratic contenders. Those already on the record backing free tuition at public colleges and universities include former Vice President Joe Biden, Sanders, Gillibrand, Harris and Warren. Estimates vary for the cost to state and local taxpayers, although Sanders acknowledged it could be $70 billion annually. Warren seemed to back away from her support for free college during an appearance in Iowa earlier in the month, however. In 2017, she co-sponsored the 'College For All Act,' which would have made tuition free at public universities. Asked in a radio interview whether she supports reducing the cost of college or offering it free, Warren responded: 'No, I think this is about reducing the cost.' It's unlikely the Democratic Party's energized base would tolerate any significant shifts to the center on free college — or any of the party's top issues. Such populist appeals helped fuel sweeping Democratic victories in last fall's midterm elections, while producing a new generation of unapologetic Democratic leaders such as New York Rep. Alexandria Ocasio-Cortez, who is aligned with the democratic socialist movement. And polls repeatedly suggest that voters support proposals for universal health care, free college and free preschool. 'We have seen a dramatic shift in the Democratic Party's political center,' said Adam Green, co-founder of the Progressive Change Campaign Committee. 'Those who deny that are hurting their chances in 2020.' Meanwhile, Castro, like others in the early 2020 field, says he's fully committed to a 'bold vision' to address the nation's top policy challenges. 'All Democrats recognize that this is not going to be easy, that to get Medicare for all, for instance, it's not guaranteed, it's not going to be easy, it may require along the way there are some compromises,' he said. 'But I'm convinced that it's worth it to go forward.