I would check out the third-party candidates. If there were one I could agree with I would vote for him or her, otherwise I would move on down to the Senate portion of the ballot.
By the way, I have never voted a straight party ticket and I have cast votes for one Democrat and one third party presidential candidate in elections past. I have voted in every presidential election since 1964 and there have only been three elections in which the candidate I voted for didn't win.
I don't like any of the current crop of Democrat presidential candidates, but would consider a vote for Richardson if his position was a little more reasonable in what he is proposing. I could not vote for him on the basis of what he has currently said he would do.