The trend toward party unity in congress can clearly be seen in voting records of members of congress when their votes are officially recorded during

- Roll call vote.

Explanation:

'Roll call vote' in simple simple terms is described as the trend when each member's vote is being recorded. Such a voting reflects the unity in a party as their votes are officially taken into account. These votes are also known as 'recorded votes' and these votes are often called when it is requested by at least one-fifth of a quorum of legislators. Such a voting reflects that how the legislators(representatives) have voted on specific grounds. Thus, 'roll call vote' is the answer.