As he made his first heavyweight foreign policy speech on Friday, the smile broke out a few times but his visage was stony and serious, his tone full of gravitas with less of the Texas twang. The leading Republican candidate for President by far, he has little reason to fear anyone else at this stage of the game, but his advisers are wary. The election is still his to lose, and they want to make sure he stays on top.
The speech had all the trappings of a presidential event. It was held in the Ronald Reagan Presidential Library in California, so the podium had the seal of the Republic on it. The backdrop royal blue curtains evoked the White House press room. Former secretary of state George Shultz introduced him and Nancy Reagan was in the audience.
The speech itself was solid mainstream realist American foreign policy, much of which the present administration would feel perfectly at home with. He distinguished himself by a far more robust attitude towards China than his Democratic opponents, calling the country "a competitor, not a strategic partner". He was firm on the need for a national missile defence, a project beloved of ageing Reagan Star Wars fans. Indeed, there was plenty of Reaganesque flourish, with appeals to principle and national interest, free trade and a solid defence.
There was a lot at stake. For Mr Bush, foreign policy has already emerged as a potential minefield. It was not just the recent embarrassment when a journalist asked him to name a set of heads of government, and he couldn't. Earlier in the campaign, he made references to Grecians instead of Greeks, and Kosovarians instead of Kosovars. Worst of all, he seemed to be in danger of losing his temper when he made these mistakes.
Mr Bush's lead, as the campaign begins to be serious, is uncontested. Nationally, he is ahead of his closest rivals, John McCain and Steve Forbes, by between 40 and 50 percentage points. But the most recent polls show that Mr McCain has almost closed the gap in New Hampshire, the first primary state, and might just deal him a big blow in the first electoral test that really matters, on 1 February.
There are rumours that Mr Bush's staff is spreading rumours about the mental stability of Mr McCain, a former naval pilot who was shot down over Hanoi. The subject is discussed under the cover of references to the Arizona Senator's irascibility. "Using the code word `temper', a group of Senate Republicans, and at least some outriders of the Bush campaign, are spreading the word that John McCain is unstable," wrote columnist Elizabeth Drew last week. "The subtext... is that he returned from five and a half years as a (prisoner of war) in North Vietnam with a loose screw. And it is bruited about that he shouldn't be entrusted with nuclear weapons."
It is on foreign policy and defence experience that Mr Bush is weakest when compared with Mr McCain, an acknowledged expert. He has a team of highly experienced advisers around him, nicknamed the "Vulcans", drawn largely from the Bush and Reagan administrations.
He has unveiled a 30-second commercial for use in South Carolina called "Dangerous World" which hammered home his commitment to national defence. A little girl wanders in an abandoned building, as aircraft drop bombs. "Today we live in a world of terror and madmen and missiles," says Mr Bush. "As President, I will have a foreign policy with a touch of iron driven by American interests and American values."
It is being aired in a state with 400,000 veterans and five large military installations - and one with a crucial place in the primary calendar. It comes just after New Hampshire, and is one of several states that Mr McCain believes he can win to dent the Bush aura of invincibility. "The McCain idea is that a victory in New Hampshire will erode the entire logic behind Bush's nomination," Christopher Caldwell writes in the Atlantic Monthly. Then "Bush's real problems will come to the fore" - that he's regarded as "a wise-guy who swears a lot and snickers, he's testy". Worst of all, "his mental calibre is now in question ... Bush's intellect is an issue".
"We always felt there would be a moment when people would start looking at us differently," said one adviser. "That moment's come, and we've got to rise to the challenge."