President Bush paid tribute Tuesday to defense workers (search) who help equip U.S. troops in Iraq and Afghanistan and also put in a plug for his administration's plans for an anti-missile system as he toured a helicopter factory involved in both missions.

In his second visit to a Boeing Co. (search) plant in a week, this time in election-critical Pennsylvania, Bush also renewed his pledge to appeal to the World Trade Organization if necessary to challenge European Union subsidies of Boeing's chief commercial aircraft competitor, Airbus.

Addressing thousands of Boeing employees and their families as well as local political supporters, Bush praised people who "work day and night to put out a good product for our country."

He spoke at an outdoor rally in a parking lot alongside a helicopter assembly and refitting plant in this Philadelphia suburb. Two CH-47 Chinook choppers (search) served as props.

Bush also spoke of his administration's proposal for an anti-missile defense system, noting that Boeing was a major contractor on the project.

"I think those who oppose this ballistic missile system don't understand the threats of the 21st century," the president told applauding workers.

The president noted that last month Boeing engineers loaded the first missile interceptor into a silo in Alaska. He characterized that as the beginning of a national shield "that was envisioned by Ronald Reagan."

Bush said opponents of the system are "living in the past. We're living in the future. We're going to do what's necessary to protect this country."

Bush's Democratic rival, John Kerry (search), said earlier this month that he would continue missile defense research but "I don't believe in rapid deployment of a system that hasn't been adequately tested."

Kerry foreign policy adviser Rand Beers said that in the months before the Sept. 11 terror attacks, "Bush and his closest advisers were preoccupied with missile defense and their misunderstanding about the threats we face continues to this day."

At a campaign rally later in Hedgesville, W.Va., Bush paused several times during his speech as the crowd of several thousand chanted, "Four More Years, Four More Years," to drown out a single demonstrator every time he shouted comments about the Iraq war and the failure to find weapons of mass destruction there.

The stop in West Virginia, a battleground where this year's race is close, was the 11th of Bush's presidency. Bush won the state 52 percent to 46 percent in 2000 over Democrat Al Gore.

Earlier, Bush noted he was making the 32nd visit of his presidency to Pennsylvania, another key battleground state this year. Gore won the state 51 percent to 46 percent four years ago.

The race in Pennsylvania this year appears extremely close, although recent polls show Kerry to have an edge. Bush has visited the state more than any other.

During his tour Bush climbed inside a helicopter that was being refitted and, at one point, used a wrench to tighten a bolt.

On the foreign competition issue, Bush said, "We're going to get rid of the subsidies for Airbus that make it difficult for Boeing to compete on a fair and level playing field."

He made similar remarks last Friday during a visit to a Boeing plant in Seattle.

Kerry said more than a year ago that the United States should subsidize Boeing just as European nations subsidize Airbus.

Responding to Bush's comments, the European Union said that it was willing to consider "disciplining" government support to Airbus — but only if the United States does the same for Boeing.