The Washington Post’s Joel Achenbach discusses Obama’s most powerful moments as an orator, and his most memorable speeches as a candidate and as president. (Joel Achenbach and Nicki DeMarco/The Washington Post)

With the right audience, in the right moment, Barack Obama is an exceptional speaker, someone who invites comparison to such masters of oratory as Ronald Reagan and John F. Kennedy. He can also be a bore.

This is the paradox of Obama’s rhetoric. He’s a spotty performer, particularly considering how good he can be when he’s at his best. He’s generally better with a script and a teleprompter than he is in extemporaneous settings, where he tends to be a cautious speaker and “high self-monitoring” in the terminology of experts. He sometimes seems to choose his words only after mulling every conceivable way they’ll be twisted and distorted by his implacable enemies.

At his very worst — and that would be during the first presidential debate last year — he can be disengaged and grumpy, like a man who’s annoyed he’s going to miss SportsCenter.

The academics who study political communication say Obama has unfinished business. They point out that, although Obama has some signature speeches — including the healing speeches after the mass shootings in Tucson and Newtown, Conn. — he still has no signature line as president, no trademark statement.

There’s no Obama utterance that’s the equivalent of, for example, Reagan’s “Mr. Gorbachev, tear down this wall!” or JFK’s “Ask not what your country can do for you . . .” He’s never produced a phrase as president that’s as memorable as what Franklin Roosevelt uttered in March 1933 in his first inaugural address: “The only thing we have to fear is fear itself.”

“I can’t think of phrases that we particularly associate with this president for this presidency,” says David Birdsell, dean of the School of Public Affairs at Baruch College. “I think that’s unusual. Even if it’s something as inartful as ‘I’m the decider’ ” — that being an off-the-cuff line from President George W. Bush in 2006 (“I’m the decider, and I decide what is best”).

For now, Obama’s most famous words include “Hope,” “Change,” “Change we can believe in” and “Yes, we can,” the buzzwords and slogans during his first presidential campaign. (All of them better than his bland re-election slogan, “Forward.”)

And, of course, there’s that riff that made him famous way back when: “There’s not a liberal America and a conservative America; there’s the United States of America.” That was his first big speech, the one in which he introduced himself to the country, at the 2004 Democratic National Convention in Boston.

From the moment he announced his presidential candidacy, his speeches were compared to rock concerts, with astonishing crowds — 5,000 people, 10,000 people, 25,000 people. With audiences like that, he had to develop a certain kind of rhetoric, the language of a man leading a political movement, long on emotion and aspiration. He’d written books, knew how to craft a good line and was able to own his words in a way that someone who relies entirely on speechwriters couldn’t.

One of Obama’s best speeches, Birdsell said, was on the night of Jan. 3, 2008, when he’d just won the Iowa caucuses — a black man running in a nearly all-white state against the Clinton machine. His tone had the cadence of a preacher in the pulpit:

“They said — they said — they said this day would never come,” Obama said. “They said our sights were set too high. They said this country was too divided, too disillusioned, to ever come together around a common purpose. But on this January night, at this defining moment in history, you have done what the cynics said we couldn’t do.”

Obama delivered perhaps his most effective speech, titled “A More Perfect Union,” on March 18, 2008, in Philadelphia, explaining his long association with the controversial minister Jeremiah Wright. His speech ranged from autobiography (“I am the son of a black man from Kenya and a white woman from Kansas”) to American history and contemporary race relations. He spoke of black anger, white resentment, stylistic differences in church worship and what he called “a racial stalemate,” all framed against his belief that Americans of diverse backgrounds share a common purpose.

Kathryn Olson, a professor of communication at the University of Wisconsin at Milwaukee, said of that speech, “The moment was right, his take was right, it had the perfect pitch of what he does well, which is reaffirm common values, share a vision in broad strokes rather in detail, de-emphasize partisanship. When he can rely on metaphors and poetic language, that’s the icing on the cake.”

Few people remember the actual words of Obama’s first inaugural address, which lacked a headline-grabbing phrase or what the professors call a “digestive statement.” The most memorable words may have been the oath of office itself, flubbed by the chief justice and leading to a do-over the next day at the White House.

“It certainly wasn’t his best speech,” Olson said. “But it didn’t matter. It wasn’t the words he spoke or how he spoke them. It was his presence there that reaffirmed the common values that he reiterated in his speech. He almost could not have failed.”

One of the toughest critics of Obama’s rhetoric is Kathleen Hall Jamieson, a University of Pennsylvania professor of communication and co-author of “Presidents Creating the Presidency.” She marks him down for a series of rhetorical failures and missed opportunities. In the category of unfortunate stagecraft, Jamieson cites the Tucson speech. Obama spoke of the 9-year-old girl who had gone to meet her congresswoman speak, only to be murdered by a madman. “I want our democracy to be as good as Christina imagined it,” Obama said. But although the words were eloquent and moving, the setting in an arena led people to applaud as if it were a rally or a sporting event rather than a somber memorial service, Jamieson wrote in the academic journal Polity.

Americans had unrealistic expectations of what Obama could achieve as an orator when he assumed office, she said.

“The expectations that were created by the press in 2008 were unsatisfiable,” Jamieson said. “The notion that the new Pericles had entered the United States discourse arena was out there. If that is the expectation, no rhetoric of governance was going to satisfy it.”

She means that, as president, Obama can’t toss around vague aspirational notions but has to talk about specific policies. His rhetoric has become more partisan, shaped by the brutal realities of gridlocked Washington. Talk of common purpose and overcoming partisan divides no longer sounds plausible.

“I recognize that times have changed since I first spoke to this convention,” the president said in September to the Democrats assembled in Charlotte. “The times have changed, and so have I. I’m no longer just a candidate. I’m the president.”

It’s possible that Obama, freed of the need to run for re-election ever again, will be looser in extemporaneous settings. He can take more chances, not just politically but rhetorically.

Like any good writer, he understands that big ideas should be simply stated, that the framing should rely on sturdy monosyllabic words (“The only thing we have to fear is fear itself”), and that silence can be an effective way to set off those ideas and allow them to breathe. For example, in announcing his proposals to curb gun violence, he discussed the Constitution and declared that with rights come responsibilities.

“Along with our freedom to live our lives as we will comes an obligation to allow others to do the same.”

He paused for three seconds, letting the words hang in the air.

“We don’t live in isolation.”

Three second pause.

“We live in a society.”

Three second pause.

“A government of and by and for the people. We are responsible for each other.”

A big idea, simply stated. He then shifted from the abstract to the specific, telling the story of one of the first-graders gunned down at Sandy Hook Elementary School.

“I’m told she loved pink.” [Pause.] “She loved the beach.” [Pause.] “She dreamed of becoming a painter.”

It wasn’t what people would typically call oratory, but it was effective.

Maybe we’ll hear the signature Obama line, the one he’ll be remembered for, in the second inaugural address. That was, historians tell us, the occasion of Lincoln’s greatest speech, one chiseled on the wall of his memorial (“With malice toward none, with charity for all . . .”). Obama knows that even though this won’t be his last defining moment, even if he has countless speeches still ahead of him, this one is for the permanent record, a chance to speak words that will remain carved into the nation’s memory long after he is no longer among us.