A new report shows that re­­pealing the stepped-up basis provision in the federal tax code, as some Democratic lawmakers have proposed, would significantly depress job creation, severely reduce the country’s gross domestic product (GDP) and put the future of family-owned farms and other businesses at risk.

The study, conducted for the Family Business Estate Tax Coalition by the consulting firm EY and released last week, showed that repealing stepped-up basis would choke off investments in...