By Bob Morgan, Jr.

With John McCain pretty much having locked up the Republican nomination for president and Barack Obama having a very clear lead among the Democrats, the candidates will have to turn very soon to their choice of vice presidential nominee.

For Mr. Obama, the challenge is to pick a candidate that will help unify the party after a rather bruising primary battle with Hillary Clinton. In theory, of course, he could pick Mrs. Clinton as his running mate, but teaming up with the former First Lady would seem counter to Mr. Obama's message of bringing change to Washington. In addition, the continuing presence of Bill Clinton in an Obama administration could be problematic.

In light of Mr. Obama's youthful age and very light experience as a United States senator for less than four years, he most likely will pick a running mate with a considerable track record in government, and preferably executive experience. The candidate will also want someone to appeal to some constituencies, like blue collar and rural voters, which have not so far been particularly responsive to his message.

While the supply of eligible Democratic governors is fairly limited, the most likely choice would be Ed Rendell, Pennsylvania's chief executive. In addition to being governor of a swing state (John Kerry only won Pennsylvania narrowly in 2004) where Mr. Obama had some difficulties in the primary season, Mr. Rendell can also boast of experience as mayor of Philadelphia.

Another possible choice would be Evan Bayh, senator from Indiana. Mr. Bayh has over eleven years of service in the Senate, including a seat on the Armed Services Committee. Before that, he was a two-term governor of Indiana. The problem with Mr. Bayh is that he may not be able to put his home state in the Democratic column. Indiana last voted Democratic in a Presidential election in 1964.

On the Republican side, Mr. McCain has his own challenges. Since the senator will be 72 years old on Election Day, his vice presidential nominee must project an image of being ready now for the presidency. Also, since Mr. McCain has only held legislative office, executive experience would rank high on the criteria for vice president.

A number of Republicans think Mitt Romney is the perfect candidate. Supporters point to his success as a businessman, in salvaging the scandal-ridden Salt Lake City Olympics and in governing heavily Democratic Massachusetts. Unfortunately, Mr. Romney also has a few drawbacks. Very unfairly, his Mormon faith has been held against him. More substantively, despite very significant campaign resources in the 2008 primary season, he never really gained traction among Republican voters; whether he will do better in the general election is questionable. Finally, despite Mr. Romney's ability to woo Democratic and independent voters in Massachusetts, Mr. Romney's conservative stances in the primary season may not help the national ticket with these voters.

Another possibility is former Arkansas governor Mike Huckabee, who ran pretty well in the primary season. Mr. Huckabee's nomination would help Mr. McCain with evangelical voters, but likely would strengthen the stereotype of Republicans as dominated by the religious right.

A very plausible alternative is the governor of Minnesota, Tim Pawlenty. Mr. Pawlenty has served as governor since 2003 and won a second term against tough opposition in 2006, generally a bad year for Republicans. At age 47, he presents a youthful (perhaps too youthful) contrast to Mr. McCain and comes from a state only narrowly carried by the Democrats in 2004.

Both Mr. Obama and Mr. McCain will have some difficult choices.