A biblical case for senior pastors?: Two questions

Most evangelical churches today have a senior or lead pastor. Can we make a solid, biblical case to establish this practice? For those who are part of a church that has both elders and a senior/lead pastor, here are two questions I invite you to answer:

Why do you have elders?

In my experience, many respond to this question with a robust, thoroughly biblical explanation of the role of elders in the local church. Our churches must have elders because of the clear teaching of Scripture, they insist. They frequently describe the normative need for elders in each church, the plurality of elders, the pastoral nature of the ministry of elders, etc.—drawing directly from clear New Testament passages—in ways that might cheer the hearts of the strongest proponents of biblical eldership. But then we ask the follow-up question:

Why do you have a senior/lead pastor?

scratching-head. . . ummm . . . . . . There can be a long pause at this point. We sense the need for a similarly robust, equally biblical explanation for this (presumably) key role . . . but it seems surprisingly difficult to find. One can rely on a pragmatic response: ‘There has to be one key leader, you know.’ Or we could resort to conjecture or speculation: ‘Well, the New Testament church had strong, prominent leaders, so . . .’  Or we can just blindly follow tradition. But I can’t seem to find anyone presenting a strong, clear, biblical case for the normative senior/lead pastor. Any takers?