The "to" of the infinitive (which is what we have here) has no meaning of its own. It is simply there to make the infinitive form of the verb ("lead" in this case). This is a headline, and as such it has been shortened by omitting the verb "to be".
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If an ordinary person says "Bush and Clinton are to lead a fund raising push", it means that they are ordering the presidents to do it
This is a headline, and as such it has been shortened by omitting the verb "to be". The full form is "Ex-Presidents Bush and Clinton are to lead a fund-raising push".