To make an intelligent choice between Hillary Clinton and Barack Obama when it comes to defeating John McCain, Pennsylvania Democrats must evaluate what the Republicans will focus on in the autumn. Past is prologue, and that's how can we predict what they will emphasize.

A little less than four years ago, the pundits were commenting on George W. Bush's low approval ratings, declaring that no incumbent president with such abysmal numbers on the eve of a presidential convention had ever been re-elected. Then the Republicans wrapped themselves in the 9/11 flag and showed buildings exploding and death and destruction in New York. After playing on the country's fears, the Republicans declared that America needed to re-elect the incumbent president during a time of war to keep the nation safe. Somehow, they managed to get the voters to agree.

Americans do not like to change horses midstream during a war. And because America is still in a war this election year, we can expect that national security again will be the battleground on which the Republicans will stake their claim to the presidency. If anyone has any doubt, they should review Mr. McCain's resume and ask themselves what is the one issue on which he is strongest: national security. Come election time, the Republicans will once again wrap themselves in 9/11, talk about terrorism and instill fear in the electorate.

If Mr. Obama is the Democratic candidate, the Republicans will regain the White House and keep the Iraq war alive even though it continues to be a horrible burden on our economy. Why? Because Mr. Obama has no credibility on national-defense issues. Unlike Hillary Clinton and John McCain, he has never served on the Senate Armed Services Committee. In a time of war, inexperience on defense issues would be the death knell for his candidacy in a general election. Mrs. Clinton has tremendous credibility on national defense, and that is why the Republicans hope Mr. Obama becomes the Democratic nominee.

Mrs. Clinton's mastery of defense issues is extraordinarily well regarded in the Senate. She has worked on health care for our troops and national guard families, as well as on 9/11 homeland-security issues. She spent eight years in the White House as the confidante and closest adviser of the commander-in-chief. She has the backing and support of 30 admirals and generals. As senator for New York, Hillary Clinton was at Ground Zero fighting for 9/11 victims and their families. She can beat both Mr. McCain and Mr. Obama on national security issues with such provocative hot-button images.

If Mr. Obama is the Democratic candidate, the Republicans will have a feast, reminding Americans of the danger in entrusting our security during a time of war to a largely unknown and untested candidate, one who associated himself with a close friend and pastor for 20 years who has preached that "America invented the AIDS epidemic for the purpose of killing African-American men."

Let's be frank: Anyone who could say something that preposterous out loud is a verifiable nut. How could Mr. Obama spend even five minutes with someone who is so off-kilter? Why did Mr. Obama continue to associate with someone spouting such race hate? Would parallel conduct by a white candidate be tolerated? If Mr. Obama did not have the good judgment to dissociate himself from the Rev. Jeremiah Wright, won't the Republicans naturally question Mr. Obama's judgment in picking a secretary of defense?

Journalists have done a disservice to Democratic voters by not vetting Mr. Obama thoroughly during the primary season. The Republican Party certainly will do so if Mr. Obama is the Democratic candidate. At that point, if Mr. Obama does not stand up to strict scrutiny, journalists will have betrayed those Democratic and independent voters who are desperate to have a high-quality, electable alternative to John McCain.

What Democrats need is a candidate who can stand up under examination and who can handle the national security debate. Journalists have failed to do a rigorous job investigating the Obama candidacy, just as they failed to subject the case for war in Iraq to close inspection. We may all pay the price in November if Mr. Obama is nominated.

Finally, the Democratic selection process is very different from the general election. In the Democratic primaries and caucuses, state delegates are apportioned basically by the percentage of the popular vote each candidate receives. In the general election, each state is winner-take-all. This means Mr. Obama cannot win the general election without winning big swing-vote states. With the exception of Virginia and his home state of Illinois, he has not fared well in such states, so his chances of beating Mr. McCain are slim from an electoral-map standpoint, as well.

The only Democratic candidate who has demonstrated a following in the states needed to win the general election -- and the only well-vetted Democrat with national-security credibility -- is Hillary Clinton.