For all the talk of party conventions as defining moments in presidential campaigns, the one that wrapped up Thursday night in Tampa underscored the degree to which they have become an outdated and ostentatious exercise of limited political value.

“This may be the last convention, in my opinion,” said former New Hampshire governor John Sununu, a prominent surrogate for GOP nominee Mitt Romney and an informal adviser to his campaign, who was participating in his seventh. “It cost a lot of money. When all the numbers are in, this will be the $100 million convention. There’s a lot of good I could do with $100 million.”

Sununu’s prediction of the demise of an electoral tradition is probably overstated, and a reaffirmation of his reputation for cantakerousness. But the sentiment behind it — that conventions are straining the limits of their usefulness — is widely shared, and GOP leaders including House Speaker John A. Boehner (Ohio) said the party might do well to follow the Democrats’ lead and scale their event back to three days.

Modern-day conventions come months after a party has made up its mind on a nominee. The real business is to give a boost to that candidate, and to spruce up a party’s image for the election.

The convention drama long ago moved from the back room to the television screen. But the Nielsen ratings estimate of the first night’s audience Tuesday was only 22.3 million viewers over seven networks, down by almost 1 million from the number who had watched the first night of the GOP’s 2008 convention. (In that year, as in this, a hurricane forced the proceedings to be delayed by a day.) And of that number, about two-thirds were over the age of 55.

The dropoff in viewership from Alaska Gov. Sarah Palin’s vice presidential acceptance speech in 2008 (37.2 million) to Rep. Paul Ryan’s (21.9 million) was even sharper, a plunge of more than 40 percent.

Still, Republicans here believed that they gained some ground, at least marginally, by giving a clearer definition to their ticket and their party.

Republicans had described the Tampa convention as an opportunity to give a national introduction to their nominee — who remains a something of a one-dimensional figure to many voters, despite the fact that he has been running for president, pretty much nonstop, for the last six years.

His wife Ann’s speech on Tuesday’s opening night was well-received, but offered little by way of new information about Romney. To that end, some of the most humanizing presentations came Thursday night, as longtime friends described a man who would help a harried mother with her laundry, assist a dying child who wanted to write a will, and believe in a business when it was still an entrepreneur’s dream.

Republicans also hoped they had laid out an effective critique that Democrats will have to answer when they meet next week in Charlotte to nominate President Obama for a second term.

Again and again from the stage, convention speakers invoked a now-infamous snippet from a recent Obama speech — “You didn’t build that” — which they contend shows his misunderstanding of how American enterprise works.

The phrase became a constant, mocking refrain from the convention stage, and delegates waved signs that said “We built it.”

So often did the statement come up that longtime Republican operative Ralph Reed said he half expected to hear it from the lips of Boy Scouts leading the Pledge of Allegiance.

“What the convention did was take what was initially a gaffe and turn it into a campaign theme — through brutal and disciplined repetition,” said Reed, who is the head of the Faith and Freedom Coalition, an organization that promotes the mobilization of evangelical voters.

At a campaign stop in Roanoke last month, Obama said: “If you were successful, somebody along the line gave you some help. There was a great teacher somewhere in your life. Somebody helped to create this unbelievable American system that we have that allowed you to thrive. Somebody invested in roads and bridges. If you’ve got a business — you didn’t build that. Somebody else made that happen. The Internet didn’t get invented on its own. Government research created the Internet so that all the companies could make money off the Internet.”

And from that unlikely moment, a convention theme was born.

Republicans also used the convention as a showcase for many of their rising stars, starting with keynote speaker Chris Christie, the governor of New Jersey.

Former Republican chairman Michael Steele described the convention as a testament to the party’s resurgence. The GOP, he said, is in the midst of a comeback that began with the elections of Christie and Virginia Gov. Bob McDonnell and gained steam with the 2010 midterms.

“It’s coming back from the abyss of voter rejection,” Steele said. “That journey reflects the party’s ability to capture the sentiment of the country and present an economic message that is resonating. Now the rest of that journey gets completed this cycle.”

But while conventions are times for slogans and broad themes, the remaining months will be more about giving voters the specifics. The real test of that will likely be in the three presidential debates.

The “convention bounce” for presidential candidates in the polls historically averages about five percentage points. And Romney advisers note that the upside for a challenger is usually greater than for an incumbent.

But as the term “bounce” suggests, it is usually a temporary event. Gauging it this year will be difficult for several reasons. Both parties’ conventions are relatively late, and separated by a holiday weekend, a time when pollsters find it hard to measure public sentiment. The September start of school may compound the difficulty.

The party activists who served a convention delegates said they came away from their four-day meeting in Tampa more enthused and dedicated to victory in November.

“It did make me more energized,” said Geraldine Davie, a retired teacher from Springfield who was part of the Virginia delegation. She had come to Tampa with a sweeping sense of purpose: “We are here to save America.”

“We’ve had a very, very good week down here. The polling is sort of all around, but the buzz out of here was very positive,” said Tom Rath, a Romney adviser. “There were several headliners any one of which could’ve carried the night and you put three or four of them together, it was pretty impressive.”

Not that this would surprise anyone, but Democrats differed with that assessment. “They’ve done their level best to put out a nice shiny package to mask the extreme content in their policies and their proposals,” said Rep. Debbie Wasserman Schultz (D-Fla.), chairwoman of the Democratic National Committee. “It just fuels the cynicism that already exists among the voters.”

No doubt that, come a week from now, the Republicans will be saying pretty much the same thing about the Democratic convention in Charlotte.

Jon Cohen, Kevin Merida and Philip Rucker contributed to this report.