Hillary Clinton pushed to expand a federal program that has cost billions of taxpayer dollars in her sweeping economic address Monday despite questions about its results.

"I'm committed to seeing every four-year-old in America having access to high-quality preschool in the next 10 years," Clinton said during her speech, highlighting a theme she has raised repeatedly on the campaign trail.

Head Start, the federal government's existing preschool program, consumed nearly $8.6 billion last year to provide the same services for which Clinton has recently advocated.

But the Department of Health and Human Services — the agency that administers Head Start — admitted in a self-published report that children who attended preschool through the program enjoyed few, if any, developmental advantages over children who did not have access to free preschool.

"There were no significant differences between the Head Start group and the control group on any measures of social-emotional development during the Head Start year or during kindergarten," said the HHS report, which was released to the public the Friday before Christmas 2012.

A 2013 analysis by the Brookings Institute found Head Start cost taxpayers $8,000 per child each year, while a federal program to fund the rest of the same child's primary and secondary education cost $700 per year.

"By third grade, Head Start had little to no effect on cognitive, social-emotional, health or parenting outcomes of participating children," a Heritage Foundation analysis of the program found.

President Lyndon Johnson touched off Head Start in 1965 with his War on Poverty. More than 32 million children have passed through the program in the decades since, costing taxpayers roughly $160 billion.

Despite Head Start's shortcomings, Clinton has frequently pushed for an increase in spending to boost taxpayer-funded preschool across the country since launching her presidential campaign in April.

She has often touted her status as a grandmother to bolster her concerns about providing affordable preschool for low-income families.