If there had been an empty chair at the Democratic convention this week, its ghostly occupant would have been Ronald Reagan.

Barack Obama admiringly referred to Reagan’s transformational presidency during the 2008 election campaign. That enraged the Clintonites, but then-Senator Obama’s take on the historical shifts in American politics was absolutely right. If you doubt that, just think back one week to the Republican convention, which was above all a coming-out party for Reagan’s 21st-century heirs.

Reagan’s legacy is so powerful because he identified the state as the central issue in American politics. That is still true today. Both in Tampa, Florida, where the Republican promise was to shrink the state, and in Charlotte, North Carolina, where the Democrats’ promise is to transform the state into a more effective servant of the middle class, the big question is what government should do, and how big it should be.

In 2008, Mr. Obama identified the force of Reagan’s leadership because he aspired to have the same impact. But the problem for him — and for American liberals in this century more broadly — is that the task they have set for themselves is both harder to do and, crucially, harder to explain.

That argument is made eloquently in a newly published essay on the Obama presidency by Theda Skocpol, a Harvard professor and one of the country’s foremost political theorists. If you read only one book about Mr. Obama this electoral season, read “Obama and America’s Political Future,” the slim volume that includes Dr. Skocpol’s essay and three smart responses. Together, they rise above the tick-tocks and polemics that characterize too much of the United States’ political writing.

Dr. Skocpol’s starting point is the comparisons between Franklin D. Roosevelt’s New Deal and Mr. Obama’s ambitious plans — often described as a New New Deal — that were often made at the beginning of his administration. But that parallel, which was drawn by insiders as readily as by outsiders, misses one crucial historical difference: F.D.R. built the state from scratch, while Mr. Obama is trying to overhaul a massive state machine that has existed for decades.

“The things that happened in the 1930s were unprecedented in peacetime,” Dr. Skocpol told me. “That was hard, but citizens could get a sense that something new was happening. What is different for Obama is that there is a very elaborated federal apparatus that already exists.”

It is the difference between a start-up and turning around a big, troubled company, or, to use a more domestic metaphor, between building a brand-new house and renovating an old one.

“We know from economic and technology history — it is easier to fill a space for the first time,” Dr. Skocpol said. “This is the same principle.”

Dr. Skocpol, who describes the Obama effort as “redirecting” the state, believes that task is difficult for many reasons. One is that change usually antagonizes vested interests; another is that, as the Obama administration has certainly shown, it often increases complexity.

When it comes to Election Day, renovation has a further, powerful disadvantage, compared with starting from scratch. When you build on a greenfield site, it is easy to show what you are doing and to explain how it is new. When you are working on a brownfield site, it is a lot harder to demonstrate what you’ve actually accomplished.

In the United States, the job of redirecting the state is further complicated by a phenomenon the author of another essay in this volume describes as the “submerged state.” Suzanne Mettler, a political science professor at Cornell University in New York State, coined the term to describe “a whole number of public policies in the United States that are hard for people to perceive as such because of their design.”

The submerged state lurks most massively in tax policy, which provides huge benefits, but ones that are largely invisible to their recipients. The result is the familiar American paradox of beneficiaries of government largess who passionately call for a shrinking of the state.

“A lot of people derive a lot of benefits from public policy, but they don’t recognize that the government is assisting them, so they are less likely to support government,” Dr. Mettler told me. She cited a survey she did in which only 43 percent of respondents admitted to ever getting help from the government. When asked about specific programs, though, 96 percent turned out to have benefited from state largess.

When it comes to the campaign trail, the easiest platform is a start-up — Americans love the shiny new thing. Next best is demolishing something that’s old and rotten — the appeal of Representative Paul D. Ryan’s radical rhetoric is no accident.

Hardest of all is to promote a painstaking, time-consuming renovation — which is exactly what U.S. government needs and what Mr. Obama, at his best, has promised to accomplish. To succeed, he needs to understand how different his new deal is from F.D.R.’s and why his transformation is a harder sell than Reagan’s was. He needs the courage to remove the cloak of invisibility from America’s submerged state. And when it is revealed to Americans in all of its complex and inefficient glory, he needs to come up with a clear plan not to make it bigger, but to make it better.

PHOTO: Former U.S. Speaker of the House Newt Gingrich and his wife Callista narrate a tribute to former President Ronald Reagan during the final session of the Republican National Convention in Tampa, Florida, August 30, 2012.  REUTERS/Adrees Latif