The presidential candidates spread messages of anxiety in Ohio and Florida on Saturday, as John F. Kerry accused President Bush of putting American lives at risk by failing to prevent the flu vaccine shortage and as Bush again said that his Democratic opponent cannot be trusted to protect the country from terrorists.

Speaking at a packed rally in a high school gym here, Kerry told Ohioans that Bush is to blame for the loss of half the nation's expected vaccine inventory for this flu season. He said the administration has forced health officials to make life-and-death decisions in dispensing dangerously tight supplies.

"We've got people standing in lines for hours on end -- some of them in their seventies and eighties -- hoping to be among the lucky ones," Kerry told the largely partisan crowd. "And every day, our health care workers struggle to make what could be life-or-death decisions as to who is going to get a shot." The Kerry campaign also released a new TV ad trumpeting this warning.

In Sunrise, Fla., meanwhile, Bush noted that this weekend marks the first anniversary of Kerry's vote against $87 billion for military and reconstruction spending in Iraq and Afghanistan -- a reminder of what the GOP calls the Democratic candidate's shifting positions on ousting Saddam Hussein. "In a time of great threat to our country, at a time of a great challenge in the world, the commander in chief must stand on principle, not the shifting sands of political convenience," Bush said.

With three new national polls, including a Washington Post tracking poll showing Bush making gains but the race still virtually tied, the two candidates have zeroed in on these two populous states as the most important battlegrounds for Nov. 2. Florida is the bigger prize, with 27 electoral votes, or 10 percent of what is needed to win the election; Ohio, historically a reliable GOP state, has 20 electoral votes. But that state has serious unemployment problems that make it too close to call.

Polls show that both candidates have a chance of winning Ohio and Florida. If one of the candidates captures both, he will likely win the White House, political strategists from both parties say.

After the rally here, Kerry took a winding bus tour of Ohio, stopping at a pumpkin patch in Jeffersonville, picking up a hunting license in Buchanan, attending a Roman Catholic Mass at St. Mary's church in Chillicothe and concluding his day at a family farm in Wakefield in the foothills of Appalachia.

Throughout the day, Kerry continued to batter Bush over the economy -- a stark reminder of how Kerry's political strategy differs greatly from Bush's focus on national security concerns. Showing a recent front-page story in a local newspaper quoting Treasury Secretary John W. Snow as saying job losses are a "myth," Kerry said the president is oblivious to reality. "Mr. President, the people who have lost jobs on your watch are not 'myths.' They are our neighbors; they are middle-class Americans," he said.

The only new twist in Kerry's speech was his attack on Bush over this season's flu vaccine, an issue grabbing headlines in newspapers across the country.

The government announced less than two weeks ago that nearly half of the country's anticipated supply -- 46 million to 48 million doses -- was contaminated, and rendered unusable, at a plant in Britain. During last week's final presidential debate, Bush said healthy Americans should forgo shots for the flu, which kills an estimated 36,000 Americans each year, to help ease the shortage. Neither Bush nor Kerry has received a flu shot this season.

According to Kerry, Bush was first tipped off about the impending crisis three years ago but refused to act. "It's just business as usual with George W. Bush: You got to ignore it, deny it, then try to hide it, and then say you would do it the same way," he said. Various groups have warned of potential shortages for years; British officials say the U.S. government was warned in mid-September of possible disruptions at the Liverpool plant.

The cause of the public health crisis is much more complicated, some experts say. Most pharmaceutical companies refuse to manufacture the flu vaccine because sales are unpredictable, profits are often minimal and costly consumer lawsuits are always possible. The United States once had multiple sources of the vaccine, but today the country has only two major suppliers, leaving Americans vulnerable to dangerous disruptions like the current one.

Steve Schmidt, a Bush spokesman, said Kerry is partly to blame because he opposed legislation in 2003 shielding vaccine-makers from some lawsuits as a way to entice more companies into the market. "Once again, John Kerry proves he has no solutions to offer, only attacks," Schmidt said.

At the rally, Kerry offered little detail as to how he would prevent future shortages, other than promising a new strategy. The Democratic nominee had not addressed the flu vaccine issue as a presidential candidate until the shortage was announced on Oct. 5, according to Stephanie Cutter, a Kerry spokeswoman. Sen. John Edwards (N.C.), Kerry's running mate, spoke in December 2003 about the importance of maintaining adequate supplies.

Bush used a day-long bus and plane tour of Florida to punch back at Kerry's assertion that the president may have an undisclosed plan to resurrect a military draft. "No matter what my opponent tells people to try and scare 'em, we will have an all-volunteer Army," Bush shouted at an outdoor rally in Daytona Beach. "The only person talking about a draft is my opponent. . . . The best way to avoid a draft is to vote for me."

However, during one appearance, he blew his line. At first, he said: "We will not have an all-volunteer Army." The crowd responded with puzzlement, prompting Bush to say, "Let me restate that: We will not have a draft."

A Democratic group backing Kerry announced a new television ad in swing states raising a warning on a possible draft.

Bush continued his aggressive appeal to his most conservative supporters, using the word "liberal" four times at a rally outside Fort Lauderdale, where his star-spangled campaign bus, with red and blue lights flashing, pulled into a darkened arena.

A Washington Post poll shows the candidates even in Florida, where early voting will begin on Monday. Bush made three stops there Saturday, and planned four more stops on Monday and Tuesday in hopes of winning the state by more than his 537-vote margin in 2000. The president was accompanied at each stop by first lady Laura Bush and by his younger brother, Florida Gov. Jeb Bush, who said he was "proud of my commander in chief, and my brother."

Edwards was in suburban Miami Saturday, kicking off the Democratic ticket's 12-city, five-day tour of the state. At a rally in Miami Gardens, he stepped up his rhetoric, saying that Republicans will be "up to their old tricks" and potentially engage in voter fraud to win the state's 27 electoral votes.

"We know what's coming. Republicans are already up to it," Edwards said to the largely black crowd, prompting a wave of boos. Edwards was responding to news reports here that suggested that Jeb Bush ignored advice to abandon a flawed election voter list before it went out to county election offices, even after he was warned by state officials that some voters -- mostly black -- could be disenfranchised

Allen and staff writer Chris L. Jenkins reported from Florida.

John F. Kerry is cheered as he arrives at a rally in Xenia, Ohio, where he used the flu vaccine shortage as an issue. "We've got people standing in lines for hours on end," he said. President Bush greets supporters in Sunrise, Fla. "The commander in chief must stand on principle, not the shifting sands of political convenience," he said at a rally.