Who could have predicted that the summer of 2009 would be such a tough time to be a liberal? Seven months ago, President Barack Obama took office with a 79 percent approval rating — the highest in three decades.

The Kennedy-esque cult of personality that surrounded the new president led many conservatives and libertarians to fear he’d be able to work his will in Congress, dramatically increasing the size of government.

Yet, cap and trade has dropped off this year’s legislative agenda and today Obama’s signature initiative — national health care — remains stalled, growing more unpopular by the minute.

A new Rasmussen poll has 54 percent of American voters preferring no health care reform to passage of what’s currently on the table. In the meantime, Obama has lost more than 25 points off his initial, stratospheric approval ratings (maybe it should have told us something that the last president to reach Obama’s levels of early popularity was Jimmy Carter).

Obama bears much of the blame for his current political woes, having pushed an overly ambitious agenda that the public seems reluctant to accept. But he’s also the victim of trends that long predated his presidency.

Back in November, political scientist Paul C. Light noted that the president’s legislative agenda had been shrinking for more than 40 years because “there is simply less room in government” for the sort of large-scale initiatives Obama has in mind.

“There will not be a New Deal or Great Society this coming year,” Light wrote — political conditions would not allow it. Among other things, the increasing prevalence of the filibuster has enhanced the Senate’s historic role as a brake on federal ambition, much to the chagrin of liberals.

But perhaps the biggest factor frustrating Obama’s legislative agenda is declining trust in government, a phenomenon that’s been on visceral display in recent town-hall meetings.

President Lyndon Johnson pushed the Great Society through Congress at a time when three-quarters of Americans told pollsters that they trusted the federal government to do what is right “most of the time” or “just about always.” Today, that number’s around 30 percent.

But for a brief post‑9/​11 blip, trust in government never again reached its Great Society-era heights after Vietnam and Watergate. Since the war in Iraq and the prescription drug entitlement passed during that brief period of high trust, we ought to be glad that it didn’t last longer than it did.

Vanderbilt University’s Marc Hetherington, the leading scholar on political trust, echoes Light, writing that declining faith in the feds makes “another Great Society or New Frontier … unlikely in a post-Cold War world.”

In fact, Hetherington sees low levels of political trust as a key factor behind the failure of the last attempt to pass national health care, in President Bill Clinton’s first term. “Opponents of health care reform caused people to think about the Clinton plan in terms of how much they trusted the government,” Hetherington writes, and this “was sufficient to seal the fate of health care reform.”

It also may do the trick this time around. Obama’s effort to assert control with one-seventh of the U.S. economy comes after nearly a year of emergency governance on the economic front.

From TARP to the auto bailout to a stimulus package that cost more than the entire war in Iraq, since last fall this administration and the last have seemed bent on fundamentally changing the federal government’s role in the economy. Americans are looking at Obamacare with that as a backdrop, and they don’t like what they see.

It’s too early to declare the death of national health care, of course, but the smart money says that Obama’s efforts will go the way of Clintoncare. Even if the era of Big Government initiatives is over, however, that doesn’t mean the era of Big Government is.

The ambitious plans of past presidents — in the form of Social Security and Medicare — will continue to increase the federal government’s share of the economy even if new entitlement programs don’t get passed.

But the resurgence of public skepticism toward federal power is good news for those of us who support limited, constitutional government. And it’s about time we had some good news.