MADISON, Wis. — Some of the nation’s signature public universities are gently prodding slackers and fifth-year “super seniors” toward the graduation stage, moving to reinvent a campus culture that treats on-time completion as the exception rather than the rule.

At 33 of the 50 state flagships, fewer than half of students graduate in four years. The overall four-year graduation rate is 31 percent for public colleges and 52 percent for private ones, the federal government reported this year.

University leaders are embracing on-time completion amid mounting concern about wasted tax dollars, spiraling tuition and America’s plummeting global rank in college attainment.

Four years and out is a long tradition at private colleges, a value reinforced by the parents who pay the bills. Public universities, by contrast, have long tolerated the five- or six-year degree. But too often, experts say, the slow track leads nowhere.

“The longer it takes people to graduate, the less likely they are to graduate, ever,” said William Bowen, former Princeton University president and co-author of the book “Crossing the Finish Line.”

Here at the University of Wisconsin at Madison, about half the students graduate on time. Nick Korger said he arrived in Madison four years ago “with absolutely no clue what I wanted to do.”

He thought about pre-med, but washed out after seeing his first biology grades. Now he is studying history and English, writing for the campus newspaper and driving a truck in the summer. He’ll return in the fall for a fifth year.

“I’m gonna go watch all my friends graduate,” said Korger, 21, of Oshkosh, Wis. “And if they’re still in the state, I hope they’ll all come watch me graduate.”

Colleges are mobilizing on the issue for several reasons. Public tuition and fees have doubled since the mid-1990s, in inflation-adjusted dollars, to an average $8,244. President Barack Obama has set a goal for the nation to regain the world lead in college attainment by 2020. The economic downturn has pushed state lawmakers to target perceived collegiate “slackers” and the tax dollars that subsidize their education.

This year, the University of Texas at Austin announced a push to raise its on-time graduation rate to 70 percent by 2016, from the current 53 percent. UT President Bill Powers said the goal is “ambitious but attainable.”

“The state pays an average of $7,563 for every student, every year,” said Texas state Sen. Florence Shapiro, a Republican. Each dawdling student, she said, “prevents another student from coming in and starting that process.”

Indiana University will offer discounted courses this summer to encourage on-time graduation. The University at Buffalo in New York unveiled an on-time graduation pledge, hoping to raise its rate from about 45 to 60 percent. Half the incoming class has signed it.

University leaders are pressing four-year graduation as a goal from the day freshmen arrive on campus. Public institutions largely avoided class-year “branding” in the past, partly to avoid shaming students who could not finish on time, and partly because on-time graduation was not the norm.

At the University of Minnesota convocation last fall, each student was given an envelope containing a “2015” tassel. President Eric Kaler told the freshmen, “Hang it where you will see it every day — in your room, on your laptop, on your roommate’s nose — as a reminder that graduating in four years is your goal.”

Other schools are working the four-year theme into recruiting events, as a selling point to the cost-conscious.

“The easiest way for me to get a round of applause is to say we expect the students to graduate in four years,” said Michael Amiridis, provost of the University of South Carolina. “And it’s not the students who applaud. It’s the parents.”

In Madison, administrators say they introduced the class year concept to 2010 freshmen and identified the Class of 2014 in a #uw2014 hash tag on Twitter. “That language was never part of our culture,” said Jocelyn Milner, an associate provost at Wisconsin.

Internal data show the university’s four-year completion rate has risen from 47 percent in 2005 to 56 percent in 2011. (The new federal data, which tracked a cohort of students that started college in 2004, put the university’s rate at 49.7 percent.)

Students arrive in Madison expecting to finish in four years, and 83 percent finish by year six. The “professional student,” who stays indefinitely to savor campus life and party on State Street, is all but extinct.

“I don’t really know anyone who’s just hanging out, eight years later, and dabbling in classes,” said Madeline Sivanich, 20, a Wisconsin sophomore from Minnesota.

Why, then, the lackluster four-year graduation rate? Students and administrators cite a tangle of academic and social factors, common to most of the nation’s flagships.

First, most Wisconsin students don’t take enough classes to finish in four years. The average student amasses 14 credit hours per semester, too few to attain the required 120 in four years. University leaders say the key problem is work. By senior year, most students hold on- or off-campus jobs.

Second, some academic programs, including majors in engineering and education, are designed to take five years. Although the American college is built on a four-year model, some programs stretch four years into five.

Third, budget cuts have led to academic bottlenecks: crushing demand for introductory courses that are graduation requirements for large numbers of students.

“What you hear every single semester is the frustration from kids. Courses fill up,” said Robert Schlaeger, 22, a senior from Milwaukee who is graduating on time.