Joe Biden¡¯s move to solidify the Democratic presidential nomination by adopting a free-college plan from his chief rival promises to benefit more than half of 7.5 million undergraduates at US public four-year institutions ¨C but experts believe that a more modest programme?would be a likelier outcome.

The plan, long backed by second-place contender Bernie Sanders and other members of Congress, also would boost enrolment at US public colleges and universities by more than 15 per cent, experts are calculating.

Such estimates, however, assume both that Mr Biden is elected in November and?has a Democrat-led Congress that overcomes the likelihood of fierce opposition from private colleges and universities.

A more likely outcome in Congress, according to policy experts, is a more modest use of federal dollars to expand existing statewide efforts to make free the first two years of public college.

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Mr Biden, who had been pushing for only two years of free college, has now? long offered in Congress by Senator Sanders that would provide four years of free tuition at public colleges for students from families with annual incomes below $125,000 (?110,000).

The plan adopted by Mr Biden would require participating states to cover some of the costs. Nearly 85 per cent of the nation¡¯s almost 7.5 million undergraduates at public four-year colleges come from families below the $125,000 limit, said Sandy Baum, a higher education policy analyst and former professor of economics at Skidmore College.

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Of them, Professor Baum said, nearly 30 per cent already don¡¯t pay tuition and fees because of existing grant aid, meaning that some 4.4 million additional students would get free tuition under the plan that Mr Biden is now backing.

Costing $41 billion a year in federal money, with another $20 billion induced from the states, the plan could be expected to increase public college enrolment by more than 15 per cent, Georgetown University analysts said.

The nation's 1,600-plus private nonprofit colleges are expected to fight hard against the proposal, which is almost certain to cut into their already shrinking enrolments.

"If you¡¯re a private college and you're not elite, you¡¯re in trouble,¡± said Anthony Carnevale, a research professor of public policy at Georgetown. ¡°Even without this, you're in trouble.¡±

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Private college lobbying, however, probably will not be enough to stop Mr Biden from pursuing the idea if elected, said Dr Carnevale, director of Georgetown¡¯s Center on Education and the Workforce.

paul.basken@timeshighereducation.com