President Bush (search) and Democratic rival Sen. John Kerry (search) vied for the senior vote Tuesday, trading jabs over Social Security and a looming shortage of flu vaccine.

With exactly two weeks left until Election Day, the two candidates were continuing to court voters in key battleground states. Bush was set to do his second bus tour in the past few days of Florida — with two of his brothers, Marvin and Florida Gov. Jeb Bush at his side. Kerry leaves Florida Tuesday for Pennsylvania and Ohio.

In St. Petersburg, Fla., on Tuesday, Bush tried to calm fears about flu-vaccine shortages that he says were caused by a "major manufacturing defect" but which Democrats link to White House incompetence.

"I know there are some here who are worried about the flu season," Bush told supporters in a stadium at a baseball training camp. "I want to assure them that our government is doing everything to help older Americans and children get their shots despite the major manufacturing defect that caused this problem."

British regulators recently shut down shipments from Chiron Corp., cutting the U.S. supply of flu shots almost in half. Democratic presidential candidate John Kerry said the Bush administration was warned about the shortage of flu vaccines three years ago and didn't act.

But "millions more will be shipped in the coming weeks," Bush assured voters. "We're stockpiling more than 4 million doses of flu vaccine for children."

In an interview aired Tuesday on National Public Radio, Kerry defended his criticism of the Bush administration for failing to heed warnings about vaccine supplies.

"If you can't get flu vaccines to Americans, how are you going to protect them against bioterrorism? If you can't get flu vaccines to Americans, what kind of health care program are you running?" Kerry asked.

Kerry also touted his own plans for preserving Social Security and cutting the budget deficit in half on Tuesday. In Florida on Monday, the Massachusetts senator encouraged supporters to vote early, hoping that would help him capture the Sunshine State's 27 electoral votes.

"Between now and Nov. 2 ... you're going to get people to those polls so we can change the direction of America," Kerry told supporters in Orlando, Fla.

Kerry's running mate, North Carolina Sen. John Edwards (search), accused Bush of failing the United States and the world in Iraq, citing unsecured nuclear weapons abroad and unprotected ports at home as further evidence of the president's "incompetence."

"He's created something that didn't exist before the war in Iraq — he's created a haven for terrorists," Edwards said at a campaign stop in Windham, N.H.

Contending that Bush gave in to chemical industry lobbyists by not requiring greater security against terrorist attacks at the nation's chemical plants, Edwards said Bush has not secured "loose" nuclear weapons in other countries and has failed to protect airports and ports in the United States.

"This is not leadership — this is incompetence," Edwards said.

Meanwhile, in Carroll, Ohio, Vice President Dick Cheney evoked the possibility of terrorists bombing U.S. cities with nuclear weapons and questioned whether Kerry could combat such a threat, which the vice president called a concept "you've got to get your mind around."

"The biggest threat we face now as a nation is the possibility of terrorists ending up in the middle of one of our cities with deadlier weapons than have ever before been used against us — biological agents or a nuclear weapon or a chemical weapon of some kind to be able to threaten the lives of hundreds of thousands of Americans," Cheney said.

"That's the ultimate threat. For us to have a strategy that's capable of defeating that threat, you've got to get your mind around that concept."

The latest Washington Post tracking poll shows Bush with 50 percent support among likely voters, Kerry with 47 percent and independent candidate Ralph Nader with 1 percent. The most recent CBS News/New York Times poll of likely voters shows Bush with 47 percent support, Kerry with 45 percent and Nader with 2 percent.

National polls show Bush and Kerry in a statistical dead heat, while the president holds a slender advantage in others. Aides to both candidates claimed to possess private polls showing their man ahead in the dozen battleground states that will determine the outcome of the election.

Early Voting Problems

Early voting is already under way in some states like Florida, Texas, Colorado and Arkansas.

During the early voting in Texas, President Bush received at least two votes — from his parents. The former president and first lady voted in Houston.

But problems are already cropping up. Accusations of voter fraud and intimidation are being thrown about across the country. There have been reports of ballot machine problems and allegations of leaving names off absentee ballots, to name a few.

In northwest Ohio on Monday, authorities released a statement on the arrest of a 22-year-old man on charges he falsified more than 100 voter registration forms in return for crack cocaine. The man said he had been recruited to collect the forms and was paid by a woman working for the state's NAACP voter drive effort. There's no evidence yet, however, that the NAACP knew of the exchange.

And it all comes at a time when the nation is almost evenly divided — giving new meaning to the words: "Every vote counts."

In the GOP stronghold of Cincinnati, officials admitted that Kerry's name was accidentally removed from absentee ballots sent to a predominantly black neighborhood. Ohio had already rejected more than 17,000 absentee ballots because they contained Nader's name, which has since been removed because he failed to submit enough valid signatures to be a candidate there.

Feel a Draft?

Meanwhile, the draft is proving to be an issue concerning young voters.

On his 14th trip to Florida this year, Bush on Tuesday tried to calm those military families in the area with a pledge not to reinstate the draft.

"We will not have a draft," Bush said in answer to Kerry's claim that re-electing the president would create "the great potential of a draft." Then, to make sure everyone understood, he later said: 'We'll keep the all-volunteer army. ... I repeat. The all-volunteer army will remain an all-volunteer army."

Kerry says re-electing Bush would create "the great potential of a draft." Not so, responds the incumbent: "The best way to avoid the draft is to vote for me."

In an election where voter turnout is the great unknown, the voting rate and preferences of 18- to 30-year-olds may be the biggest wild card on Nov. 2.

"One of the things that have been puzzling us is how young voters will behave on Election Day," said Andrew Kohut, an independent pollster at the Pew Research Center. "They've been pretty volatile, sometimes strongly in Kerry's camp and other times driven back to Bush."

Young voters were evenly divided between Bush and Kerry in polling conducted this month by Ipsos-Public Affairs for Newsweek.com and The Associated Press. Among likely voters under age 30, Kerry led 52 percent to 42 percent.

The University of Pennsylvania's National Annenberg Election survey found that half of young people believe Bush wants to reinstate the military draft. Only 8 percent said Kerry wants it.

The day the poll was released, Bush said in his second debate with Kerry, "We're not going to have a draft, period."

A week later, the Democrat told The Des Moines Register: "With George Bush, the plan for Iraq is more of the same and the great potential of a draft."

Campaigning in a Florida community dependent on the military, Bush fired back Saturday: "We will not have a draft ... The best way to avoid a draft is to vote for me."

In an interview Monday with the AP, Bush accused Kerry of scare tactics and insisted he would not bring back the military draft, even if there were a crisis with North Korea or Iran.

"I believe we've got the assets and manpower necessary to be able to deal with another theater should one arise," Bush said.

Special-interest groups backing Kerry are fueling rumors of a draft in a second Bush term. MoveOnStudentAction.org is launching a nationwide campus "Feel a Draft?" campaign to demand an exit strategy in Iraq and urge Bush to detail a specific plan to avoid the draft.

Kerry has argued that a "backdoor draft" exists because some U.S. forces have been required to extend their military careers to serve in Iraq.

FOX News' Mike Emanuel, Major Garrett, Liza Porteus and The Associated Press contributed to this report.