TAMPA -- When Mitt Romney stands before the Republican National Convention and the nation Thursday night, he has to come across as both old Mitt and new Mitt.

Before he started his presidential journey five years ago, Romney was regarded as an easy conversationalist and collegial guy. Since then, he has come off as cautious, perfectly pressed, and prepared, a man who can make even a moment such as his “surprise” embrace of his wife on the convention stage after her speech Tuesday seem cold and scripted.

Romney’s mission is to appear as warm as his wife painted him in her address that evening. He needs to slay the image that Democrats have been crafting for months — and he has inadvertently helped along — that he’s an insensitive, out-of-touch business executive with little feel for the plight of those who aren’t in his economic or social class.

The pre-campaign Romney enjoyed engaging others, even having fun at his own expense. In 2004, he broke into song with the Kingston Trio when he was introducing a new kind of transit-system fare card in downtown Boston. They sang Charlie on the MTA,’ a tale of a man who never returned from a trip on the system because he couldn’t afford the “one more nickel” he needed to exit. Three years later, Romney spent the first 10 minutes of a McClatchy interview happily talking about American Motors cars from the 1960s.

But the candidate on the stump in 2008 and in recent months has been unusually careful, his words measured, his tone serious and even solemn. Efforts at candor have produced odd-sounding observations that stand out all the more against the focus-tested campaign boilerplate. “The trees are the right height,” he said on one trip this year to his native Michigan. “The streets are just right.”

Chances are Romney will be tightly scripted Thursday, as he faces the same kind of challenge that historically confronts presidential candidates: Their convention speeches are their unfiltered introductions to the American people, the chance to leave voters with the unwavering impression that they’re ready and qualified to be the president of the United States.

At the same time, though, they have to evoke enough empathy to make those same voters think that they understand and care about them.

“Make a strong, powerful speech, but show your personal side,’’ veteran Republican consultant Bill Dal Col said.

“Look competent but human,’’ added Tobe Berkovitz, a former Boston-based Democratic media consultant.

Looks aren’t a problem. Fit, handsome, graying at the temples, Romney is a stage-ready vision of a president. “If you were casting a movie, you’d pick him because he looks the part,” said Sal Russo, a veteran Republican political consultant.

The words have to match the look. He has to be both visionary and precise. Romney has been criticized at times on the stump for being corny with his expressions of patriotism. He likes to quote — and sometimes sing — America the Beautiful.

Romney’s speeches often list what he sees as failures of the Obama administration. Earlier this month, he told a Chillicothe, Ohio, audience, “You have not been forgotten. We will not leave you behind.” He segued to his own policy discussions by saying, “It wasn’t supposed to be this way.”