We measure our presidents against not only our hopes for the present, which are sometimes unreasonable, but also our understanding of the past, which can be just as flawed.

Has a misreading of history informed a misappraisal of Barack Obama?That’s a question raised, not explicitly but implicitly, by a new book by the Princeton historian Julian Zelizer, “The Fierce Urgency of Now,” to be published on Thursday.

Its setting is the 1960s, as the title, a phrase uttered by Martin Luther King Jr., suggests. Its focus is Lyndon Johnson. And one of its conclusions is that despite Johnson’s legend as a peerless legislative tactician, he was largely a hostage of Congress and of forces beyond the presidency.

Zelizer reminds us that many of Johnson’s signature victories came during a two-year period when Democrats had two-thirds majorities in both the Senate, where they held 68 seats, and the House, where they held 295.

Zelizer also reminds us that Johnson’s trouncing of Barry Goldwater in the 1964 election spooked Republicans to a point where many fought progressive legislation less stridently than before, lest they be portrayed as Goldwater-style extremists.

Those dynamics and others worked powerfully to Johnson’s advantage, and when the climate and the Congress changed, so did his fortunes. On the domestic front (as well as on the foreign one), the final two years of his presidency were a bust, at least in comparison with what preceded them.

Obama’s name appears just twice in Zelizer’s book. But it’s impossible not to think of him more often, given how frequently the yardstick of Johnson’s presidency has been applied to his.

If only Obama were a schmoozer like Johnson. If only he had Johnson’s taste for the muck of lawmaking. If only he had Johnson’s patience for minutiae.

Zelizer told me that when he began work on the book more than five years ago, “I still had a kind of view of Johnson, as many do, as someone who really knew how to work the system. What’s surprising to me is that as conditions in Congress change, he is really shut down. I didn’t expect the last part of the book: a president who’s really emasculated and can’t get anything done even though he’s trying the same old tricks. It really became crystal clear to me how Congress determines the fate of the presidency.”

Republicans currently control both chambers, and have a House majority bigger than before. That bodes disastrously for Obama’s legislative dreams, and it’s the point of reference for his impulse to wield executive authority.

Zelizer said that instances over the last 50 years of a president truly imposing his will on a Congress fully or partly controlled by the opposing party are rare. Ronald Reagan got tax cuts in 1981 despite a Democratic majority in the House, but he’d just shellacked Jimmy Carter in the 1980 elections and Democrats were running scared.

Johnson’s name is popping up a lot now. This year is the 50th anniversary of many of the laws grouped under the Great Society, and the movie “Selma” is drawing complaints for its portrayal of Johnson as resistant to voting rights for blacks and sharply antagonistic to King.

“It’s not fair to Johnson,” Zelizer told me.

But in his view, Johnson has been considered too kindly by writers who attribute the Great Society to his wizardry.

“He was cagey, he was smart, he was politically savvy,” Zelizer said. “But that doesn’t explain why the bills passed.”

And Obama can indeed be cold and disengaged. But, Zelizer said, that’s not why he hasn’t scaled the legislative heights that Johnson did.

Johnson benefited from “a vibrant period for grass-roots mobilization as a result of the civil rights movement,” he said, adding that there was pressure for legislation from the bottom up, which is most effective.

There hasn’t been any commensurate mobilization during Obama’s presidency. Zelizer said that voters frustrated with congressional inertia should examine their own exertions . and the ways in which campaign financing, lobbying and gerrymandering have created a dysfunctional legislative branch . as much as any president’s character.

Zelizer’s read on things leaves ample room for Obama to be questioned on foreign policy and for not making more of his first two years, when Democrats controlled Congress. It’s also possible that he should have made less of them, that delaying health care would have spared Democrats their 2010 drubbing and given him additional time with a friendly(-ish) Congress.

But it’s undeniable that we treat our presidents as larger than life, simplifying the stories we tell. They’re not always mighty frigates parting the waters. They’re just as much buoys on the tides of history, rising and falling with the swells.