The left's pressure on centrist Democrats has steadily grown since the health care debate began but MoveOn.org and Democracy for America today launched the most direct threat yet: together, they've secured commitments from 66,000 members to donate a total of $3.5 million to support primary challenges against any Democratic senator who joins a Republican filibuster to block an up-or-down vote on health care reform. The groups announced the commitment in a fundraising email to supporters today.