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(CNN) —  

Higher education has become a focus of the ideas primary waged on the left flank of the Democratic presidential primary.

There are important distinctions between plans put forward by Sens. Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts, but they share twin goals of making public higher education largely free and forgiving most or all of the more than $1.5 trillion in student debt currently held by Americans.

There are also multiple proposals to help with education for children by raising teacher pay (Sen. Kamala Harris of California), providing free lunches (Sanders and former HUD Secretary Julian Castro) and providing free or subsidized child care (Warren). Sen. Cory Booker of New Jersey has an interesting proposal to give every young child a nest egg they could use on higher education. But it is the proposals from Sanders and Warren that may get the most attention, particularly among the young voters Democrats hope will turn out.

How to cancel student loan debt

Sanders unveiled a plan for mostly free tuition before the presidential campaign, but Warren was the first to propose massive debt forgiveness. Sanders has now joined her, but there are key differences in their plans.

Warren would forgive some debt, she says, for 95% of the people who have it. It would focus on people at lower incomes, giving $50,000 in debt forgiveness to every person with household income under $100,000.

The benefit would then slide by $1 for every $3 additional of income. Someone in a household with $160,000 would get $30,000 and someone in a household with $250,000 would get none. The main target of this benefit would be debt held by the US government, but she says private debt would also be eligible. She says the total cost of the onetime benefit would be $640 billion.

Sanders’ plan is less progressive, more generous and more than twice as expensive. He’d enact a onetime cancellation of the $1.5 trillion to $1.6 trillion in student debt held by all Americans.

He says the plan would address racial and gender inequality, because women and racial minorities tend to hold more student loan debt, and would help struggling Americans with as much as $3,000 per month that’s currently going to make payments on student loans.

Both Warren and Sanders would increase spending on Pell grants for non-tuition expenses and funding for schools that target minority students, like historically black colleges and universities.

Free tuition

In addition to canceling debt, Sanders and Warren would seek to make public school tuition largely free.

Sanders would pay $48 billion per year to cover two-thirds of tuition at two- and four-year public colleges and universities. He’d put new requirements on public schools to qualify for the money.

Warren would similarly pair with states to split the cost. Not all states might take part, as occurred with the Affordable Care Act, when the government partnered with states to expand Medicaid coverage. In that case, the federal government covered more of the cost.

Other Democrats have less generous proposals.

Opposition among Democrats to free college

Some Democrats have rejected these types of proposals.

“As a progressive, I have a hard time getting my head around the idea of a majority who earn less because they didn’t go to college subsidizing a minority who earn more because they did,” said South Bend, Indiana, Mayor Pete Buttigieg, who owes more than $130,000 in student loans, back in April.

Other critics agree it’s unfair for the people who have diligently been working to pay loans off but would see no benefit under such programs. Buttigieg would rather expand Pell grants to help low-income students.

Sen. Amy Klobuchar of Minnesota is also opposed. She would do things like freeze loan rates and, like former Vice President Joe Biden, has backed free community college.

Biden has suggested support for free college in the past and includes free community college in his own proposal, but he does not have an announced plan for four years of free tuition.