US presidential candidate Barack Obama is to air a half-hour, prime-time advert on seven US TV channels.
The move comes after his Republican rival John McCain, campaigning in Florida, launched an attack on Mr Obama's national security credentials. In a speech in Tampa, Mr McCain made it clear he thought Mr Obama unqualified to act as commander-in-chief.
Mr Obama is also in the key battleground state, for a rally later with former President Bill Clinton. Six days from the 4 November election, Senator Obama leads in national and most swing-state opinion polls.
Mr Obama's half-hour TV spot will be shown on CBS, Fox and NBC, at a cost of about $1m (£630,000) per network, as well as on Spanish language channel Univision, BET, MSNBC and TV One.
Mr McCain has also been attacking his rival's defence and security policies
During one portion of the spot, Mr Obama will speak live from a rally in Florida.
The BBC's Justin Webb in Washington says that while it is not unheard of for American politicians to buy large chunks of television time - millionaire Ross Perot did it in 1992 - Mr Obama's move is unprecedented in its scope.
Only one of the major TV networks, ABC, is not running the film - which has been weeks in the making - and Fox News has decided not to broadcast its pre-game show ahead of the fifth game of the baseball World Series in order to accommodate it.
Earlier on Wednesday, in a speech to supporters in Tampa, Mr McCain questioned his rival's security credentials, saying: "The question is whether this is a man who has what it takes to protect America from Osama bin Laden, al-Qaeda and the other great threats in the world."
"He has given no reason to answer in the affirmative".
Mr McCain's assault on his opponent's defence and security policies came after a meeting with a number of senior former military officials.
At an earlier event in Miami, Mr McCain had launched a series of attacks on his rival's tax policies.
"This is the fundamental difference between Senator Obama and me," he said.
"He thinks taxes are too low, and I think that spending is too high."
Mr Obama, at an event in North Carolina, poured scorn on Mr McCain's recent accusations that he is a "socialist".
"By the end of the week, he'll be accusing me of being a secret communist because I shared my toys in Kindergarten," he told supporters.
After his TV spot airs, the Democratic candidate is scheduled to join Mr Clinton for an evening rally in Kissimmee, near Orlando, timed to coincide with the late evening news shows.
The latest Associated Press-GfK poll has Mr Obama and Mr McCain virtually tied in Florida and North Carolina. It has Mr Obama leading Mr McCain in Colorado, Nevada, New Hampshire, Ohio, Pennsylvania and Virginia.
The latest Gallup poll has Mr Obama leading Mr McCain nationally by 50% to 43%.
From: BBC NEWS