Learn English – good alternative to “low man on the totem pole”

From Wikipedia:

Vertical order of images is widely believed to be a significant representation of importance. This idea is so pervasive that it has entered into common parlance with the phrase "low man on the totem pole." This phrase is indicative of the most common belief of ordering importance, that the higher figures on the pole are more important or prestigious. A counterargument frequently heard is that figures are arranged in a "reverse hierarchy" style, with the most important representations being on the bottom, and the least important being on top. Actually, [among Native American totem poles], there have never been any restrictions on vertical order -- many poles have significant figures on the top, others on the bottom, and some in the middle. Other poles have no vertical arrangement at all, consisting of a lone figure atop an undecorated column.

Regardless of the origin, the term "low man on the totem pole" is generally understood to mean LEAST important. Using it to mean most important would probably just lead to confusion.

You could say that a person or issue is a bottleneck in the project.