You may think that the engine of change in American politics at the moment is the Republican freshman caucus in the House. Its members have reshaped the budget debate, brought an end to congressional earmarks, assaulted Obamacare, spawned a new debate on environmental policy and made a federal case over the Joint Strike Fighter engine. They’ve made a difference.

But they’re not the biggest engine of change in American politics. The governors are.

A generation ago the nation’s governors stressed imagination and competence. From their group emerged four presidents, each with a distinctive record in his state capitol. To a man they were optimists. Indeed the only presidents between John F. Kennedy and Barack Obama who were unalloyed optimists were the governors, two Republicans and two Democrats: Jimmy Carter, Ronald Reagan, Bill Clinton, George W. Bush.

Today’s governors are a different breed. As a group, they face challenges perhaps greater than any of their predecessors since the Great Depression. They can’t govern the way earlier governors did, transforming their offices into laboratories of democracy, mostly through spending. General-fund expenditures in the states declined by 7.3 percent in fiscal 2010, according to the National Governors Association, which predicts that spending and revenue won’t return to pre-recession levels until as late as 2014.

Part of the change in the character of the nation’s governors is ideological. Republicans outnumber Democrats 28 to 22, and today’s Republicans are far more conservative than their predecessors, who sometimes (Nelson A. Rockefeller of New York, Tom McCall of Oregon, Francis W. Sargent of Massachusetts) leaned pretty far toward liberalism.

A dramatic example: Maine’s last Republican governor, John R. McKernan Jr., was regarded as a moderate and is married to Sen. Olympia Snowe, one of the last moderate Republicans on Capitol Hill. Today Maine’s Republican governor, Paul LePage, has ties to the tea party and has signaled he won’t embrace the state’s longtime bipartisan consensus on environmental issues.

But much of the change is economic. There may be signs of recovery in the private sector, but not in government, where states face severe budget challenges, lingering pension problems, persistent Medicaid obligations and growing infrastructure obligations. The most depressing reading in the winter of 2011 isn’t some treacly romantic tragedy in the paperback racks at the airport. It’s the state-of-the-state messages delivered by the nation’s governors.

Some excerpts from these documents of despair, all delivered by Democrats:

• “Governors across this nation, both Democratic and Republican, have been forced to propose budgets that either raise taxes during tough economic times or set thousands and thousands of children, families and seniors adrift from programs they have depended upon for years. Education budgets are being slashed. Prisons are closing. Health care programs are being stripped.” — Gov. Mike Beebe of Arkansas

• “California faces a crisis that is real and unprecedented. Each of us will have to struggle with our conscience and our constituencies as we hammer out a sensible plan to put our state on a sound fiscal footing, honestly balance our budget and position California to regain its historic momentum.” — Gov. Jerry Brown of California

• “Just like families across Colorado do at their kitchen tables, we have to sit down at this kitchen table and make difficult choices in order to pay our bills, manage our expenses and live within our means in obviously leaner times. How we bring our budget into balance and put Colorado on a sustainable fiscal path will be perhaps the most important legacy of this legislative session.” — Gov. John Hickenlooper of Colorado

• “This is a time of crisis for our state.” — Gov. Andrew M. Cuomo of New York

None of this is to say that it has ever been easy to be a governor. Former Gov. Judd Gregg of New Hampshire, who also served in the U.S. House and Senate, regards the governor’s office as the most difficult job in politics. In his years in Concord, from 1989 to 1993, five of the state’s seven largest banks failed, two public utilities went bankrupt, the only major military base in the state was closed, and state tax revenues fell for the first 12 months.

“Indeed, gone are the days when governors undertook creative new initiatives that cost money. Republican Mitt Romney, who served as Republican governor of Massachusetts from 2003 to 2007, pioneered a state health care plan that not only would be inconceivable today but also could be a burden to him if, as expected, he seeks the GOP presidential nomination next year. His plan has striking similarities to the Obama health plan that is anathema to many conservatives, especially the tea party activists who likely will be a major force in Republican primaries and caucuses.

And though Republican Gov. Scott Walker of Wisconsin has drawn attention for his campaign against the state’s unionized workers, the budget he submitted early this month will likely prove to be another bitter battleground. He is calling for state spending to be cut by more than $4 billion over the next two years, including more than $1 billion in aid to schools and local governments.

After the Republicans took over Capitol Hill in 1995 for the first time in four decades, House Speaker Newt Gingrich invited GOP governors William F. Weld of Massachusetts and John Engler of Michigan to a House Republican caucus. They explained how they cut spending, were hanged in effigy in their states and then were re-elected.

“Our message was that you can be fiscally conservative and still be re-elected,” Weld said in a conversation this month. “The lesson from that came from the governors. It may again.”

— David Shribman is executive editor of the Pittsburgh Post-Gazette.