I'm one of those constituents, and it is true he ran on a pro-life platform, and he had the endorsement of the Virginia chapter of National Right-to-life.
The Democrats attacked his Republican opponent for being pro-life during this campaign. Thus showing that Democrats have no real convictions, only the desire for power. They ran anti-pro-life ads against the opponent of a democratic pro-life senator.
That actually made my Democratic senator mad, and it could be that anger that made him do what was right. After all, he also promised to oppose gas tax increases and in-state tuition for illegal immigrants, but he ended up voting for both of those.