The U.S. presidential election campaign has entered a new phase, with both major party candidates criss-crossing the country in search of votes in the wake of this week's Democratic National Convention in Boston.

After ceding the week to John Kerry and the Democrats, President Bush got back out on the campaign trail Friday with a stop in Missouri, one of several key swing states in the November election.

Public opinion polls indicate more Americans believe Mr. Bush is better able to handle the threat of terrorism than Senator Kerry, and the president was quick to make that point on Friday.

"You see, you cannot talk sense to the terrorists," he said. "You cannot hope for the best. You cannot negotiate with them. We will engage those enemies around the world, so we do not have to face them here at home."

The president has begun a campaign swing through several so-called battleground states in the Midwest, including some of the same states that John Kerry will visit over the next several days.

At their convention, the Democrats and Senator Kerry made it clear that they will not back away from the security issue between now and the election on November 2.

"I defended this country as a young man, and I will defend it as president," he said. "Let there be no mistake, I will never hesitate to use force when required. Any attack will be met with a swift and certain response."

With the Democratic convention now over, attention will focus on the Republican convention scheduled for late August in New York City.

But before then, political analyst Jack Pitney of Claremont-McKenna College in California says, the newly-nominated Democratic ticket of John Kerry and John Edwards will hit the campaign trail, hoping to build on the momentum they picked up from their convention in Boston.

"You will see both Kerry and Edwards criss-cross the country, do a lot of traveling, a lot of campaign events," he said. "And you will see them getting endorsements from officials, and trying to stake out more specific positions on the issues. It is a bit of a danger there, because the more specific you get, the more you open yourself up to attack from the other side."

President Bush and Vice President Dick Cheney will also be making campaign swings through a number of key states before their convention begins.

Analyst Jack Pitney says the Democrats' generally positive tone at their convention could have an impact on the Republicans when they gather in New York.

"Well, first they need to get ready for a good convention," he said. "The Democrats had a pretty good convention, and Republicans need to maintain a positive tone at theirs. They need to stay out on the campaign trail, and hope that events on the international scene and the domestic economic scene work to their favor."

The general election campaign will begin in earnest after the Republican convention concludes on September 2. The next major events in the campaign cycle will come in late September and early October, when a series of debates will be held, three for the presidential contenders and one for the vice presidential candidates.

With polls showing the race very close, most experts believe the relatively small pool of undecided voters probably will not make up their minds until just before the election, making this year's debates a crucial stage in the election process.