Pete Buttigieg, who is running for the Democratic presidential nomination in part on his experience with national service as a Navy Reserve officer who deployed to Afghanistan, on Wednesday proposed a major expansion of voluntary public service programs that aims to attract 250,000 Americans in the near term and potentially grow to one million a year by 2026.

His plan calls for expanding existing national service organizations like AmeriCorps and the Peace Corps and also adding new ones focused on combating climate change, treating mental health and addiction, and providing caregiving for older people. The new programs would prioritize bringing volunteers into predominantly minority communities and rural areas.

In an interview, Mr. Buttigieg, the 37-year-old mayor of South Bend, Ind., noted that only a fraction of applicants to AmeriCorps and the Peace Corps are accepted every year for available positions. According to government data, the Peace Corps currently has about 7,300 volunteers and trainees, while AmeriCorps has about 75,000 members.

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If elected, Mr. Buttigieg’s campaign said work on getting 250,000 Americans enrolled in these expanded programs would begin on his first day in office, an expansion which would be more than triple the size of those existing programs. A spokeswoman for his campaign said doing so would cost approximately $20 billion over 10 years.