Tuesday's primaries in the delegate-rich states of Texas and Ohio could be decisive in the race for the Democratic Party's presidential nomination between Senators Barack Obama and Hillary Clinton.  One of those two will face Senator John McCain, the presumed Republican Party nominee, in the general election.  The outlines of that campaign are already starting to take shape, as we hear from VOA National correspondent Jim Malone.

The main issues in the campaign have been clear for sometime, the state of the national economy, the war in Iraq and national security.

But each of the main candidates will try to shape the debate in ways favorable to them.

For Democrat Barack Obama, that means making change a central theme of the campaign.

"At a time like this, the American people need real change," he said.  "The kind of change that is more than about just switching parties in the White House.  We need a change in our politics, somebody who can help close the divisions in Washington so we can stop just talking about these challenges and actually start solving them."

Obama's rival, Hillary Clinton, has emphasized her experience in the Democratic race, and would do so again in a general election match-up with Republican John McCain.

"Whoever sits at that desk in the Oval office on January 20, 2009, needs all the tools available, all the resources at our disposal, and the wisdom to know how to use them," she said.  "I propose a new strategy to restore our moral authority, end the war in Iraq and defend and protect our nation."

McCain has argued that he has more experience than either Democrat, especially on national security.  His status as a war hero and the only one of the three contenders to have served in the military could prove to be an asset in the general election campaign.

"My campaign will be based on a vision of fixing the problems and challenges that face America's economy and education," he added.  "I will be very glad to stand on my record of support for not surrendering in Iraq, and my knowledge and background on national security that I can keep this nation safe."

Public opinion polls suggest a close election in November no matter which Democrat winds up facing McCain.

But McCain could face some powerful political headwinds that experts say could give the Democratic candidate an advantage.

Norman Ornstein is a political analyst at the American Enterprise Institute in Washington.

"The context of this election, in significant part, is a very unhappy country," he explained.  "A country that believes the wheels are coming off in a lot of different places.  They are unhappy about the economy and the war, which includes an acknowledgment that things have gotten better, but still a belief that we never should have been there in the first place."

McCain could also be running against history.  Americans have a tendency to want a change in direction after one party has held the White House for two or more consecutive terms.

The Democratic candidate, whether it is Obama or Clinton, is likely to focus on the need for change after eight years of Republican President George Bush.

Experts say McCain's best argument might be that his age and experience would be an asset in a dangerous, unpredictable world.

Stuart Rothenberg publishes a political newsletter in Washington.

"The election in part is going to be about change and leadership," he said.  "It is going to be about experience.  People may not think about those as issues in the traditional sense, but they certainly are, because when you elect a president, you are electing a person to run the country, really.  And individual public policy matters are important, but leadership and strength and experience and change tend to be even more important issues."

History has shown that trying to predict now what will be the decisive factors in the campaign months down the road can be foolhardy.

As analyst Norman Ornstein says, there is always the unexpected.

"Let us face it, in an environment that is not a particularly good one for Republicans, if there is a terrorist attack or a hint of a terrorist attack, if the agenda shifts to the basic theme that the barbarians are at the gate and who do you want in the house protecting the family, that is not a very good comparative ground for Barack Obama," he noted.  "It is slightly better one for Hillary Clinton were she to win the nomination."

The Democrats will formally nominate their candidate at their national convention in Denver in late August, while the Republicans will do the same in Minneapolis-St. Paul in early September.

The presidential election will be held November 4.