cross a/the red line

To defy or violate the furthest limit of what is tolerable, allowable, or forgivable. The government warned that the neighboring country's preparations of weaponry and troops crossed the red line of their longstanding treaty. The senator made it clear that implementing a tax increase of any kind would be crossing a red line for him and his party. What has happened to collaboration in politics? These days, if you merely consult with members of the other party, it's like you've crossed a red line!