Contentious as President Barack Obama’s second-term agenda may be in other respects, there’s one item that shouldn’t be: expansion of early childhood education.

Whether it’s the $7 return on investment that Obama cited in his State of the Union address or more shown by some studies — and others, less — it’s clear that high-quality pre-kindergarten programs pay dividends in lower costs for prisons, welfare, drug abuse and other troubles.

Anyone on Capitol Hill who questions the potential benefits — or others just interested — can attend a mind-bending presentation at 9 a.m. Wednesday at SVC201 in the Capitol Visitor Center hosted by a bipartisan group of 12 lawmakers.

I’ve seen the show and it’s dazzling. Drs. Patricia Kuhl and Andrew Meltzoff have a machine at the University of Washington — a magnetoencephalograph — that records the real-time brain activity of young children.

Their pictures show — graphically and dramatically — that babies who are read to, held and talked to by their parents develop thick synaptic connections in the brain — the basis of future intelligence, school performance and life success—and those who aren’t, don’t.

The process continues through the toddler years. As Meltzoff says, “Children learn more in the first five years of life than they will in any other five-year period. Enrichment given to them then can have a major effect on their performance in school, their chances of finishing high school and going on to college. The return on investment in early childhood education is truly profound.”

Republicans in Congress may well be skeptical of the returns on some of Obama’s other proposed “investments” — and even more so of his claim they will not add a cent to federal deficits — but they ought to take notice of the decisions of Republican-tilting states’ governors on pre-K.

It was no accident that Obama cited Georgia and Oklahoma in his State of the Union address and went to Georgia to promote quality early education for every child.

Those states have model pre-K programs. Georgia furnishes pre-K to 61 percent of its 4-year-olds and the state has raised its standards, requiring all state-supported teachers to have a bachelor’s degree.

According to a review by the New America Foundation, Oklahoma has 73.5 percent of its 4-year-olds enrolled in half- or full-day preschools and teachers need both a bachelor’s and certification in early childhood education.

One study showed that gains in the cognitive abilities of poor children in Tulsa’s Head Start and state preschool programs were on par with the most-effective programs in the country that may return as much as $10 in social savings per $1 invested.

Other states with more than 40 percent enrollment include Florida, West Virginia, Texas, Arkansas and South Carolina.

And Republican governors who made preschool a priority in their 2013 agendas include Georgia’s Nathan Deal, Indiana’s Mike Pence, Wisconsin’s Scott Walker and Michigan’s Rick Snyder.

Is that an argument for just letting the states manage — and pay for — early childhood education? It’s not. States are budget-strapped and need federal help.

Nationwide, only 28 percent of 4-year-olds and 4 percent of 3-year-olds were enrolled in state programs.

Another 11 percent of 4-year-olds and 8 percent of 3-year-olds were enrolled in federally funded Head Start programs for low-income children and around 30 percent of children were in private programs, which vary vastly in quality.

A quarter of 4-year-olds and half of all 3-year-olds attend no preschool at all. Some may be in the care of attentive caregivers, but plenty of others are sitting in front of TV sets, which do nothing for brain development, the University of Washington team has found.

According to the Organisation for Economic Co-operation and Development, the U.S. ranks 28th out of 38 major countries in the percentage of children enrolled in quality pre-K programs.

Just as preschool education has been promoted on a bipartisan basis in the states, it has also been advanced by both the George W. Bush and Obama administrations.

Bush began pushing to overhaul Head Start and Congress’ 2007 reauthorization of the program called for half of its lead teachers to have bachelor’s degrees by this year and established a “recompetition” procedure for agencies to win Head Start contracts.

Obama established a “race to the top” competition for state preschool programs as well as for K-12 and, as part of the Obamacare law, created a home-visit program to teach new mothers how to teach their children as well as look after their health.

Unfortunately, Obama has not said how much his program would cost or how he would pay for it — and the “high quality” preschools he correctly cited as paying dividends are expensive, at least $8,000 annually per child, according to Lisa Guernsey of the New America Foundation.

That would mean, to cover all 4-year-olds, spending $10 billion to $15 billion more than the $9 billion currently being spent.

Then again, the average annual cost to house a prison inmate is more than $20,000 and two-thirds of U.S. prisoners haven’t finished high school.

And quality of preschool programs — meaning trained teachers, low pupil-teacher ratios and good curriculums — is important.

“We want to make sure that the rhetoric about pre-school does not lead to lazy policy,” Guernsey wrote in 2009. “To generate returns on investment, we must not simply increase the number of children enrolled in existing programs.

“We need to raise the quality of these programs [and] failure to do so could undermine support for early education.”

So Congress should scrutinize the Obama proposals for effectiveness, but those who doubt that they could make a difference should show up Wednesday. They’ll be amazed.