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.