This month's Republican primary debate covered a wide range of issues from Donald Trump's insults toward women to social issues. But one big topic went completely without mention: student loan debt.

Over the course of two hours, just one candidate — Florida Sen. Marco Rubio — mentioned student loans. Rubio, who only paid off his own loans four years ago, referenced his past $100,000 debt to make the point that Democratic frontrunner Hillary Clinton wouldn't be able to “lecture” him about living paycheck to paycheck.

Otherwise, the nearly $1.2 trillion and growing, student debt load millions of Americans carry went unmentioned.

See also: How Democrats want to make college more affordable

For Republicans looking to boost their appeal to young voters and middle-class Americans, that could be a problem: Student debt is no longer simply a “young voters” issue that politicians can afford to give short shrift. The total debt has become so large and so spread across age groups that experts say it’s stifling economic growth and making income inequality worse nationwide.

The data shows how the problem has grown. A decade ago, more than 42% of all student debt ($346 billion total at the time) was held by people under the age of 30, according to research from the New York Federal Reserve.

But by 2014, less than third of the total debt (now $1.15 trillion overall) was held by borrowers younger than 30. The largest share (33%) was held by those between the ages of 30 and 39. And almost a fifth was held by people in their 40s.

The soaring debt looks even more staggering when you chart it across age groups.

In 2004, borrowers under 30 had $146 billion in student loans, compared to $114 billion for those in their 30s and $49 billion for those in their 40s. By 2014, though, the totals had more than doubled for the two youngest age groups, and increased four-fold for those in their 40s, a sign of how just long the debt can linger for many borrowers.

If the last few days are any indication, it seems the lack of attention paid to student loans a week ago won’t last. Clinton, for her part, rolled out a student debt relief plan on Monday, and even enlisted emojis later in the week to help make her case. As The New York Times put it Friday, the plan “has put pressure on her opponents as she has made a centerpiece of her agenda an issue that resonates across classes, party lines and generations.”

Other Democrats have also addressed the issue head on. Former Maryland governor and Baltimore mayor Martin O’Malley’s plan calls for reducing costs, refinancing loans and tying payments to income. Vermont Sen. Bernie Sanders, for his part, has called for making higher education free entirely.

But it’s still unclear how Republicans will respond. Bush blasted Clinton’s plan almost as soon as it was released. He called it “fiscally irresponsible” and said, “We don’t need more top-down Washington solutions that will raise the cost of college even further and shift the burden to hardworking taxpayers.” He went on to say the U.S. needs “pro-growth economic policies that will give more college graduates the opportunity to achieve earned success,” but so far hasn't released a detailed plan of his own, let alone one addressing costs.

Rubio, meanwhile, called the plan “Obamacare for College” and went on Fox News to criticize it. For the most part, his plan involves greater “transparency” around student loans, helping people be aware of the costs of college before attending. But he’s also added little about tackling cost.

Heather Jarvis, a student loan expert, said the lack of detailed proposals isn’t likely to continue for long, for a very simple reason: “Student loans affect many, many Americans, and it’s because tuitions have risen so sharply,” she said. “Only the very rich can afford to pay for college with cash.”

“I think the candidates are going to start talking about it a lot more," she added.