"Horses and bayonets" became the most memorable catch phrase of the debate between Barack Obama and Mitt Romney on Monday night (Tuesday NZT), as the Democratic United States president reached far back into the past to paint the Republican's foreign policy ideas as outdated.

Romney has criticised Obama's military policy throughout the campaign, accusing the president of spending too little to strengthen the military by noting that the US Navy now has fewer ships than it did in 1917.

The former Massachusetts governor made the point again during the final debate of the presidential campaign, and Obama pounced.

"You mentioned the navy, for example, and that we have fewer ships than we did in 1916," Obama said. "Well, Governor, we also have fewer horses and bayonets, because the nature of our military's changed.

"We have these things called aircraft carriers, where planes land on them. We have these ships that go under water, nuclear submarines," he said.

Obama even evoked a children's military role-playing board game, Battleship, to bash his rival.

"The question is not a game of Battleship, where we're counting ships," he said.

The strength of the US Navy is a particularly important issue in Virginia, which is home to some of the navy's largest shipbuilding and repair operations and is one of the politically divided "swing states" likely to decide the November 6 election.

Both men have campaigned intensely in Virginia, which has the highest level of defence spending of any US state per capita, providing the state with about 900,000 jobs. The Hampton Roads area in southeast Virginia has the largest concentration of military bases and facilities of any metropolitan area in the world.

Romney held fast to his stance that the navy needs more ships.

"The navy said they needed 313 ships to carry out their mission," he said. "We're now at under 285. ... I want to make sure that we have the ships that are required by our navy."

'PATRONISING'

Obama's "horses and bayonets" comments provided the latest debate-related phrase to become popular on social media. The hash tag "horsesandbayonets" trended on Twitter, where the mock user name @horsesandbayonettes was quickly born.

A new tumblr website was created with entries such as a picture of Obama captioned, "We also have fewer bows and arrows and catapults" and images of Romney riding a horse and carrying a gun with a bayonet.

The fuss was reminiscent of how those on social media spoofed the Sesame Street character Big Bird after the first presidential debate on October 3, when Romney said that he would cut government funding for public broadcasting even though he loved Big Bird, one of the network's most beloved children's characters.

Another catch phrase, "binders full of women" took on an Internet life of its own after the second presidential debate on October 16, when Romney used the awkward expression to describe receiving the resumes of women candidates for Cabinet positions when he was governor of Massachusetts.

Romney's Republican backers have dismissed the trends as trivialities that Obama supporters have used to try to distract from the Democrat's brief slide in the polls after Romney outmaneuvered him during their first debate.

On Monday, they called Obama's reference to horses and bayonets unnecessarily dismissive of Romney. A Republican "instant response" group posted a video of the exchange on the Internet under the headline: "Obama with the Patronizing Line of the Night."

But Democrats were delighted, and Obama aides praised the president's tough talk.

Massachusetts Senator John Kerry, who helped Obama prepare for the debates, jumped on the Battleship line, even using a catch phrase from the game to mock Romney.

"I think (Obama) just sank Romney's battleship," Kerry, who is widely viewed as a potential successor to the departing Hillary Clinton as secretary of state if Obama wins a second term, wrote on Twitter.

- Reuters