Which part of the senate is up for election at any one time answers?

Senators have 6-year terms. There are Senate elections every 2 years (the even years) where about 1/3 of the Senate is up for election. The Senators from the same state are never up for election at the same time. But during any 6 year period, both of your Senate seats will be up for election.

This is just one reason why voting in the non-presidential years is so important. The other important reason is that all members of the House of Representatives are up for election every two years. Unfortunately, the off-presidential election years usually have low voter turnout, especially among the poor and the young. This exacerbates the fact that the Senate and the House often do not represent the true makeup of the country.