Races in North Carolina, Minnesota, and Mississippi are key
...and that would not be good for us as a Party!
It is clear that once a party gets "too many" seats in Congress they feel as though they don't have to work across party lines. This leads to the extremes pushing agendas that they have been working on for years. With a filibuster proof Senate there won't be any easy excuses not to do many things. Also, the Republicans would find it easier to band together to oppose Democrats. The end result is insular thinking and a lack of motivation to really think hard. It is why Republicans became corrupt so quickly in the 1990's and lost their political acumen.
It would be great if we could get 60 seats and be a serious party, but I doubt it. The right number is probably about 56 or 57. This way, a few senators on either side would potentially have the leverage to acheive good results.