The list of candidates is half what it was before the Nov.2 election, when prominent Democrats such as former Treasury Secretary Robert Rubin were considered hot prospects had Sen.John Kerry won the White House.

名单上的候选人人数只有11月2日选举前的候选人人数的一半，当时像前财政部长罗伯特鲁宾这样的著名民主党人被认为是热门候选人，如果参议员约翰凯利入驻白宫的话。

本句中的when从句为非限制性定语从句，修饰the Nov. 2 election. When从句本身是个主从复合句。其中had sb won the White House为省略了if 的虚拟条件句。有趣的是，表面上看这个句子的主句谓语为陈述语气，但条件从句却是虚拟语气，好像不符合逻辑。其实这是因为hot prospects前省略了不定式to be的缘故。if虚拟条件从句并不是修饰were considered, 而是不定式to be的条件状语。如果改为主动句，就可以看到if从句对应的主句的虚拟语气了。

...the Nov. 2 election, when prominent Democrats such as former Treasury Secretary Robert Rubin were considered (to be) hot prospects if Sen.John Kerry had won the White House.

...the Nov. 2 election, when most people considered that prominent Democrats such as former Treasury Secretary Robert Rubin would have been hot prospects if Sen. John Kerry had won the White House.