In 2012, Tea Party's Mistakes Compounded IRS Abuse
Whatever role the IRS played in suppressing the Tea Party during the 2012 election, I think that Tea Party supporters have to acknowledge that the movement made its own mistakes that prevented it from becoming a real force in the presidential election. Chief among these was failing to identify leaders who could carry the movement through a new, governing phase and who could challenge Romney effectively in the GOP primary.
The Tea Party deferred to the new Republican leadership in the House of Representatives, failing to steer the policy agenda but setting itself up to be blamed for most of the leadership's mistakes. In addition, the Tea Party lacked a coherent means of responding to repeated abuse by the mainstream media over the Tucson shootings and other incidents. There was also lots of local infighting, lots of personality clashes.
Overall, the Tea Party suffered the same problem that plagues the nation as a whole: lack of leadership. Obama may not have led the nation, but at least he was sure to lead the small faction that comprises the most active part of the Democratic Party donor and voter base. Romney led Republicans, but could not lead conservatives to turn out in the numbers he needed--and likely would not have, in retrospect.
That does not reduce the importance of the IRS scandal even if it was not responsible for tipping the election. Watergate did not hand the election to Nixon; he would have trounced McGovern anyway. The principle is what matters--the separation of ruling party and state--and if the administration or the Obama campaign is found to have any direct hand in it, there should be severe political and legal consequences.