When it comes to writing speeches, the Republican who would be president of the United States is a tinkerer.

And not just a tinkerer. Mitt Romney is a deadline-pushing tinkerer whose mantra is that no speech is finished until after it’s delivered. Romney takes extensive notes in his journal, peppers his aides with questions about specifics and transmits speech versions back and forth to his advisers late into the night.

It’s become a joke among some of Romney’s senior aides that the worst job on the campaign is to be Romney’s speechwriter. In a campaign organization that has grown exponentially, it’s speechwriting in which Romney’s inner micro-manager reveals itself.

An English major in college, Romney is a voracious reader and is particular about the words he utters, advisers say. He obsesses and fine-tunes, for speeches consequential and trivial, on airplanes and in hotel suites. A business executive close to Romney said the candidate approaches speechwriting as he would constructing a persuasive essay.

So it is that as Romney prepared to deliver the most important speech of his political career Thursday night at the Republican National Convention, he spent months reading past nominating and inaugural speeches (including President Obama’s) and biographies. By the middle of last week, as the guts of the speech were coming together, he asserted, in a conversation with an associate, “I still have to write it.” On Friday, Romney told talk radio host Hugh Hewitt, “Mine is still a work in progress, kind of early stage.”

Over the weekend, Romney took two days off the campaign trail to finish his drafts and rehearse with teleprompters at his New Hampshire getaway home. When reporters asked him after one rehearsal for a sneak peak of his speech, Romney previewed just five words: “Thank you, ladies and gentlemen.” He was laughing, but advisers said the rest was in fact still subject to change.

On Wednesday, advisers were chiming in on this line or that line. One of them said that Romney will keep tinkering until just before he steps onto the Frank Lloyd Wright-inspired convention stage shortly after 10 p.m. Thursday – because, well, “he just likes to tinker.”

Romney’s chief political strategist, Stuart Stevens, has been at his side all week. Stevens helped Romney’s wife, Ann, write her widely lauded convention speech on Tuesday night, and he is doing the same for Romney.

Romney has an expanding speechwriting shop that helps him prepare his remarks, including Lindsay Hayes, a one-time speechwriter for Sarah Palin, and Matthew Scully, a former speechwriter for President George. W. Bush, who also worked on Palin’s convention address in 2008. But the big speeches fall under Stevens’s purview, and he and Romney have developed a rapport trading versions between their iPads.

Romney and Stevens have been writing the convention speech together, aides said, although Stevens insisted this week that Romney wrote the speech himself.

Stevens points to Romney’s June 2011 campaign announcement address in New Hampshire as a model for the message he will deliver through Election Day. That speech was a defense of free enterprise, individualism and American power in the world.

“It’ll be a clear vision of a Romney presidency and very much from his heart about America and why he wants to be president and what a Romney presidency would be like,” Stevens told reporters.

But Romney’s address in Tampa, which Stevens estimated will take about 40 minutes, will be twice as long as almost any speech he has delivered in this campaign, and the stakes are enormous.

Mark McKinnon, a top strategist on the Bush and McCain campaigns, said Thursday night’s speech will be Romney’s “most important moment of the campaign.”

“It will be his best and perhaps last real-world opportunity to fill in the blanks and correct misperceptions about him in a way voters and the media find compelling, authentic and relevant,” McKinnon said. “The reality is that it’s pretty hard to give a bad convention speech. On the other hand, it’s not easy to deliver a truly great one, and that’s what Romney needs to do.”

Romney, advisers say, sees his task as introducing himself to a national television audience of millions — both as a competent leader with the real-world experience and business know-how to turn around the economy and as a trustworthy, committed and warm-blooded family man.

Romney’s supporters said they have high hopes for him, noting his track record of rising to the occasion at the most high-pressure moments. When he arrived in Florida for that state’s critical primary, wounded after a loss in South Carolina, Romney put on the best two debate performances of his campaign and vanquished former House speaker Newt Gingrich at the polls. A month later, when he faced a do-or-die primary in Michigan against former senator Rick Santorum (Pa.), Romney grinded out a win in his native state.

“Mitt is a Peyton Manning, I think, and loves the big moment,” said one of his top fundraisers, who is close to the candidate and requested anonymity to speak candidly about him. “There are some people that always deliver in the fourth quarter when you are behind and have two minutes left.”

It is not unusual for Romney to tinker with his speeches only minutes before he delivers them. Earlier this month in Ohio, Romney was set to deliver a speech outside the stately town square in Chillicothe. The speech would wrap up his four-day bus tour, during which he announced Rep. Paul Ryan (Wis.) as his running mate, and the campaign released excerpts in the afternoon focusing on Medicare.

But as Romney’s motorcade made its way across the Buckeye State that afternoon, Romney and Stevens were aboard the campaign bus crafting new lines to add on a completely different subject. Minutes before Romney was to deliver the speech, his campaign released a second batch of excerpts – including some of the harshest rhetoric of his campaign.

And as night fell on Chillicothe, Romney thundered, “Mr. President, take your campaign of division and anger and hate back to Chicago and let us get about rebuilding and reuniting America.”