Julián Castro, whose campaign for the Democratic presidential nomination has so far struggled to gain much momentum, unveiled a proposal Monday to create a national, federally funded prekindergarten program, eliminate tuition at public universities and community colleges and alter the student-debt repayment process, part of a sweeping policy plan that his campaign hopes could reshape public education.

Under the plan put forth by Mr. Castro, the former housing secretary and San Antonio mayor, borrowers would pay nothing until they earned at least 250 percent of the federal poverty level. The plan also includes investing $150 billion to update school facilities for pre-K through high school and imparting a federal tax credit that would increase teacher pay by up to $10,000 per year.

He is proposing paying for the plan, which his campaign estimates will cost upward of $1 trillion to implement, by repealing the Trump tax cuts and raising tax rates for corporations and high-income earners.

“Chronic underinvestment in our schools, teachers and students over many decades and at all levels has allowed our competitors to leave us behind, and made an already unequal system more inequitable,” Mr. Castro wrote in announcing the plan. “Our antiquated commitment to a K-12 system,” he said, “has left our students and nation at a disadvantage.”