The expiration of  the assault weapons ban in 2004 is directly related to the shooting of the  26 students and teachers in  Newtown CT with an AR 15 semi automatic assault rifle.
The  reason why there has been  no  legislation to ban assault weapons   during the Obama administration , despite a string of mass shootings,  is likely due to  the chilling effect of outside spending on political campaigns by the NRA. This is the direct result of the 2010 Citizens United Supreme Court decision.
According to Opensecrets.org,  in 2012 the majority of  money spent by the NRA was  on  outside spending  campaigns for or  against politicians.  In 2012  the NRA spent $19 million in outside spending campaigns for or against political candidates, while   $1 million was spent in direct contributions to political candidates.
This included  outside spending of  $11 million  against Democratic candidiates, $2 million for democratic candidates, $6 million for  Republican candidates, and $500,000 against republican candidates.
The list of candidates the NRA engaged in outside spending  campaigns AGAINST  in 2012 included: Barack Obama  $8 million, Sherrod Brown $700,00, Tim Kaine $500,000, Bill Nelson  $400,000, Claire Mccaskill $300,000, Tammy Baldwin $300,000, Richard Lugar $200,000.
Even though most of the democrats the NRA spent outside money opposing won, and a majority of the republicans the NRA spent money for  lost, the outside  spending was highly effective.
Democrats and Republicans alike  clearly understand that those who would introduce   legislation the NRA finds objectionable  will  face a massive outside spending campaign to  unseat them.
As a result of  Citizens United, corporations  such as gun manufacturers  and interest groups such as the NRA  can engage in unrestricted outside spending campaigns against  politicians, allowing them  to wield outsized influence in  Congress and obstruct legislation.
These corporations and  interest groups  can now engage in political extortion simply by having the ability to wage  outside spending campaigns against politicians who would enact legislation they perceive to be unfavorable.
Citizens United has enabled these entities to hold law makers hostage, which is why  we have no meaningful gun legislation, no meaningful  banking reform and no meaningful health care reform.
Law makers are averse to introducing  legislation which might offend   corporations .This is the lesson learned from the defeat of Richard Lugar and is  likely the reason why the assault weapons  ban being proposed by Diane Feinstein has 900 exceptions in it.
Thanks to Citizens United, the NRA, which represents gun manufacturers, dictates  national gun  policy, health insurers dictate  health policy and wall street dictates  banking policy.
Citizens United has shifted the balance of power between business and government, undermining the ability of Congress to legislate. This is the culmination of a deliberate 40 year effort to undermine effective government by conservatives, including the Supreme Court.
The NRA is little more than an astro turf group for gun manufacturers , who use  the second amendment  as an excuse to  obstruct  any legislation  which might interfere with the lucrative  profits obtained from the sale of semi automatic assault weapons.
We now  have 26 dead  students and seven teachers  because no legislation was passed to ban assault weapons following  prior mass shootings during  Obama’s first term.  This is likely the result of the NRA’s ability to engage in outside spending , thanks to Citizens United.