What does "rank" mean here?

From Cambridge dictionary: a row, especially of people or things standing side by side The idea being that a party should show a united front and stay in line with their party. Breaking rank means leaving that united front and going against what the rest of your party is doing. It's often used in a more literal sense of a row of soldiers in formation, then breaking rank (either running away, or if they're going to do something else).

a rank can also refer to a formation of people/other things (generally side by side in a line). That's the usage here, but it is metaphorical. it is effectively saying that they "broke out of formation" by voting against a bill proposed by their party/allies.

It has the same meaning in a relatively common saying "Join the ranks of \_\_" which is a dramatic way of saying "Join" or "Become a member of"

See noun definition 3: [https://www.merriam-webster.com/dictionary/rank](https://www.merriam-webster.com/dictionary/rank)

Soldiers stand in a defensive line, called a rank. They are united. If someone leaves the line, they "break rank", leaving a space for enemies to attack.