Hillary Clinton and President Obama get barbecue at Midwood Smokehouse after Tuesday's campaign rally in Charlotte. (Melina Mara/The Washington Post)

Updated: 10:25 a.m.

Hillary Clinton is significantly broadening a proposal to offer debt-free college to more students as part of an effort to appeal to young voters, including many who supported rival Bernie Sanders in the Democratic primaries.

Clinton would eliminate college tuition for working families under the proposal she is announcing Wednesday. The new proposal borrows a key principle from the Sanders college plan, but it stops short of the free-college-for-all plan he offered to great acclaim from young supporters.

Before Wednesday, Clinton had proposed that community colleges be tuition-free for all working families. Her plan also sought to dramatically reduce the debt incurred by students at four-year institutions through a variety of steps.

The original cost of Clinton's college plan — which she said aims to help students "drowning in debt caused by ever-rising college costs" — was $350 billion over 10 years. A Clinton aide said the expansion will raise the cost by more than $100 billion. Any such plan would require congressional approval.

The plan was applauded Wednesday by Sanders, who said at a morning news conference outside his Washington campaign office that it could "revolutionize" higher education.

"The dream of higher education will become a reality for all, regardless of the economic circumstances of their family," he said.

The Clinton plan would ensure that families with income below a certain level will pay no tuition at in-state public colleges and universities, according to her campaign. The plan ultimately would cover more than 8 in 10 families. The expansion of the tuition plan would be introduced on a sliding scale: At the start, students from families making $85,000 a year or less could attend a four-year public college or university tuition-free. The income threshold will increase by $10,000 a year every year over the next four years, the campaign said, meaning that by 2021 all students with a family income of $125,000 or less could pay no tuition.

Clinton is also proposing a three-month moratorium on the repayment of federal student debt immediately upon taking office. And she is proposing that federal aid called Pell grants be available to students year-round instead of only during the school year.

Clinton’s also plan won immediate praise from the Progressive Change Campaign Committee, a grass-roots group that has lobbied for debt-free college.

“This represents a doubling-down by Hillary Clinton on the idea if you’re a student in America, you should be able to attend your state’s public colleges and universities and graduate with zero debt,” said Adam Green, the group’s co-founder, adding that the plan “should be welcome news for Bernie Sanders supporters.”

Sanders proposed that free tuition be available to all students attending public colleges and universities. He has proposed paying for his plan through a new tax on Wall Street speculation.

During the primary battles Clinton called Sanders's plan unworkable and unaffordable, often saying that she didn’t think that the taxpayers should foot the bill for Trump’s children to attend college. But young voters overwhelmingly backed Sanders over Clinton, a problem that potentially spells trouble for her come November.

In a poll of 18- to 29-year-olds by Harvard's Institute of Politics this year, 54 percent of young voters viewed Sanders favorably compared with 37 percent who viewed Clinton favorably. And a study released in June by Tufts University found that during the 2016 primaries, Sanders won more votes among those under age 30 than Clinton and Trump combined.

Clinton, aware of her challenge with the younger demographic, is planning a number of targeted outreach efforts in coming months, including stepped-up use of celebrity validators.

College affordability was a major topic during a high-profile meeting Clinton held with Sanders last month, and the two strategized about ways to promote the issue during the general election and in the Democratic Party platform, a statement from Clinton's campaign said. Aides to to the two Democratic presidential candidates have been negotiating a unified plan in recent weeks.

Sanders has been pushing Clinton to adopt several of his campaign policy priorities as a condition for endorsing her. While Clinton has effectively clinched the nomination, Sanders remains an active candidate.

Asked at his news conference what other ideas he was seeing movement on from Clinton, Sanders praised the education idea again, then said "we are working with Secretary Clinton on some initiatives."

The moratorium on repayment of federal student loans in Clinton's new plan would allow time to refinance or restructure high-interest debt.

Clinton addressed the problem of high student debt during remarks to a convention of the nation's largest teachers' union on Tuesday. Clinton said that she would allow teachers and other graduates who enter public service fields to refinance their student loans.

"I want everyone to be able to refinance your student loans so you never have to pay more than you can afford and for people who go into public service, and I include teaching because it is the first and primary public service," Clinton said at the National Education Association meeting in Washington. "Any remaining debt after you refinance will be forgiven after 10 years."

Student borrowers could defer loan payments for three months under a reprieve she is promising to impose through the unilateral power of the presidency.

Clinton is campaigning Wednesday in Atlantic City to draw attention to her campaign's claims that Republican opponent Donald Trump has cheated workers on his way to business success. Trump operated several casinos in the city, but filed for bankruptcy protection.

Clinton is framing her general-election case against Trump as a choice between someone who tries to address problems such as student debt and someone she accuses of selfish motives.

On Tuesday, Clinton promised to work hard for voters as she campaigned alongside President Obama. Neither Democrat directly addressed news that the FBI would not recommend criminal prosecution over security lapses involving Clinton's private email system while she was secretary of state.

David Weigel contributed to this report.