Plusses:
1. Personal Chemistry
There can be little doubt that this is the main reason why Obama is looking at Kaine. The first state governor outside Obama's Illinois to endorse Obama for President, Kaine was immediately drawn to the Illinois senator because they are two peas in a political pod. Both Harvard Law graduates with Kansas roots, both attorneys with a central focus on civil rights, and both relatively new to the big leagues, Obama and Kaine clearly like one another and enjoy each other's company.
Obama has realized that a modern Vice President practically lives with the Chief Executive, especially in times of crisis, and he wants someone he can trust completely. One other similarity that some see is less flattering. Obama and Kaine are both supremely self-confident, and their adversaries (and even some of their friends) occasionally detect a hint of hubris and arrogance. On the other hand, we have never known a President or governor who was genuinely humble. That special quality possessed by the meek doesn't go hand in hand with high political office.
4. Virginia
For decades, analysts prematurely proclaimed that the Old Dominion had become the New Dominion, and in the twenty-first century it's finally true. Among the most improbable of 2008's toss-up states, Virginia is on the knife's edge. It is not unreasonable to expect the sitting Governor to add a couple of points to Obama's total. Kaine is popular (mid-50s in most surveys), though not wildly so, as was his predecessor, Mark Warner, who is currently cruising to a big U.S. Senate victory in the state.
Oh, it almost goes without saying that John McCain would have a very difficult time finding the 13 electoral votes he might lose in Virginia. Keep in mind that Virginia has voted Republican in thirteen of the last fourteen presidential contests (save only LBJ's in 1964), and the state is tied for the best GOP record in the nation. Even Georgian Jimmy Carter, who won all the other states of the South, couldn't take Virginia. Defeat in the New Dominion would be a major and perhaps decisive blow to McCain. Could Tim Kaine be the first VP nominee since Lyndon Johnson in 1960 to deliver a critical, toss-up home state for his ticket?
Big Minuses:
4. The Aftermath in Virginia
Naturally, Obama would not be especially concerned about the post-Kaine era in Virginia, and given the frustrations of his governorship, one could hardly blame Kaine for grabbing a chance to move up and out--and eventually perhaps have his own shot at the presidency. However, many Virginia Democrats are privately unhappy at the prospect of Kaine leaving in mid-term, potentially the first Virginia governor not to complete the single four-year term since it was established beginning in 1852. That is because Kaine would be succeeded by a deeply conservative Republican lieutenant governor, Bill Bolling.
For complete story
http://www.rasmussenreports.com/public_content/political_commentary/commentary_by_larry_j_sabato/vice_president_tim_kaine