SUNRISE, Fla. (AP) _ Democratic presidential candidate Barack Obama plunked down $4 million for a campaign-closing television ad Wednesday night and summoned voters to "choose hope over fear and unity over division" in Tuesday's election. Republican John McCain derided the event as a "gauzy, feel-good commercial," paid for with broken promises.

"America, the time for change has come," Obama said in the final moments of the unusual ad, a blend of videotaped moments and a live appearance before thousands in Sunrise, Fla.

"In six days we can choose an economy that rewards work and creates jobs and fuels prosperity starting with the middle class," Obama said.

The 30-minute ad, aired on CBS, NBC, Fox and several cable networks, came days from the end of a race in which Obama holds the lead in polls nationally and in most key battleground states as he bids to become the first black president.

And while it is unusual for candidates to acknowledge the possibility of defeat, Republican running mate Sarah Palin said she intended to remain a national figure even if the ticket loses next week. "I'm not doin' this for naught," she told ABC News in an interview.

Republicans and even some Democrats said the race was tightening as it neared the end. Although Obama made no mention of McCain in his paid television ad, both men sharpened their rhetoric during the day.

McCain, in Florida, argued that Obama lacks "what it takes to protect America from terrorists" as he sought to shift attention away from the economy.

"The question is whether this is a man who has what it takes to protect America from Osama bin Laden, al-Qaida and the other great threats in the world," he said. "He has given no reason to answer in the affirmative."

Obama, in North Carolina, said if, "Sen. McCain is elected, 100 million Americans will not get a tax cut ... your health care benefits will get taxed for the first time in history ... we'll have another president who wants to privatize part of your Social Security."

For weeks now, the race has tilted Obama's way as the two men traverse traditionally Republican states — Obama angling for a sizable triumph and McCain hoping to win the White House in a close finish.

Associated Press-GfK polls taken within the past several days showed Obama ahead in four states that supported President Bush in 2004 and essentially even with McCain in two others. A separate survey suggested even McCain's home state of Arizona was not safely in his column.

The 30-minute campaign commercial, purchased at a cost that campaign aides put at roughly $4 million, not only marked Obama's attempt to seal his case with the electorate, but also underscored his enormous financial advantage in the race. He has outraised McCain by far after first committing — and then reneging — on a pledge to limit spending to the $84 million available under federal matching funds.

Obama used his commercial to pledge a rescue plan for the middle class in tough times. "I will not be a perfect president," he said. "But I can promise you this — I will always tell you what I think and where I stand."

Across 30 minutes, the ad blended views of Obama speaking in a setting that resembled the Oval Office, at the Democratic National Convention and elsewhere as well as scenes of Americans discussing their economic and health care troubles. His wife, Michelle, and his two daughters had cameos, and there were photos of his black father from Kenya and white mother from Kansas.

Obama wrapped up his day with a rally in Kissimmee, Fla., co-starring Bill Clinton. "We have so much promise and so much peril," the former president told the crowd. "This man should be our president."