I think it demonstrated that it is hard to predict what will happen in an election. There are many factors to consider, and it is easy to get caught up in the big picture and lose sight of the smaller one. You cannot assume that winning the presidency will mean winning senate or house seats for your party, and vice versa.
I have to concur with other editors above in pointing towards a variety of factors that have led to the senate being "in play" as you describe it. I don't necessarily think you can just point to Obama and his Presidency with his new policies to explain this change. I think #4 makes a very valid point concerning the role of the economy and how this has impacted so many things, not just the role of the Senate.
All of that being said, the Senate isn't really "in play" as your question suggests. The Republicans will certainly gain seats next week, perhaps as many as six or eight, but almost no one is arguing that they can take control of the Senate at this time. The House is another story completely.
The four races you mention, in states where Obama or his appointees were taken out of their Senate seats, would certainly or at least probably have been safe seats if Obama hadn't been elected. Regardless, however, voters are likely to blame whatever party is in power when economic times have been bad for a while. If the Republicans do take the House and/or the Senate in this election, voters may well turn on them in 2012 if the situation with the economy does not markedly improve. So I would argue the fact these specific seats (which are usually safe for the Democrats) are in play has much more to do with the economy than with Obama's election.
So are you asking us to tell you how Obama winning in 2008 led to the Senate being "in play" this year? If so, here goes:
First of all, the party that wins the Presidency almost always loses seats in Congress in the midterm election that happens two years later. (See stats in the link.) This happens because people generally get mad at whichever party is "in power" and so they vote against it two years later.
Second, Obama's particular initiatives (when combined with the bailout at the end of Bush's second term) have led to the formation of the "Tea Party." These people feel that government is getting way too big and is spending way too much.
So the Senate being "in play" is due to a combination of factors -- one that is constant and one that is specific to Obama.
After typing the above, I saw the rest of what you typed (in your "answer"). That stuff is just saying that Obama winning took 3 seats that (they are saying) would have been solidly Democratic and made them less solid because the incumbents were no longer holding them (because they'd become Pres, VP, and Secretary of Interior).
Senate Democrats are in danger of losing their majority on Nov. 2, and two of the reasons go back to Barack Obama's own election as president: Illinois became vulnerable after he vacated the seat to move to the Oval Office, and the same thing happened in Colorado when he appointed the incumbent Democrat, Ken Salazar, to be his Interior Secretary.
It could have been worse. The Delaware seat, long held by Joseph Biden before he became vice president, looked to be in big jeopardy until the GOP primary yielded a wild-card candidate in the person of Tea Party favorite Christine O'Donnell. New York is not on the list because Republicans could not find a strong candidate to take on Kirsten Gillibrand, who was appointed to the Senate seat when Hillary Rodham Clinton became Secretary of State. dont understand