The army of labor organizations that helped President Obama win the White House in 2008 have seen their spending power drop off dramatically since then, not only because of the unions' shrinking membership but because so many other outside interest groups are this year spending more money than ever before.
Labor union spending on political campaigns accounted for 35 percent of all outside spending in 2008, when interest groups spent a total of $87 million. But unions account for just 14 percent of outside spending this year, which has already reached $174 million, according to data from the nonpartisan Center for Responsive Politics.
The shrinking political power of traditionally Democratic labor unions coincides with the rising influence, on the other end of the ideological spectrum, of billionaire businessmen and corporations that now spend three times as much as the labor groups.
That shift in spending and influence is attributable to the 2010 Supreme Court decision in a case known as Citizens United in which the court lifted restrictions on political spending by labor unions, corporations and wealthy individuals.
The court ruling led to a surge in spending by so-called Super PACs, groups that can raise and spend unlimited amounts of money for political purposes - often without revealing the names of their donors - as long as they don't coordinate directly with candidates' campaigns.
Since that ruling, union contributions to outside groups remained relatively steady, but overall spending by outside groups of which unions are just a part has more than doubled, according to the Center for Responsive Politics.
The industries fueling the surge of outside money include the financial, real estate and insurance sectors, the center found. Businesses that gave just over $6 million to outside groups during the last election cycle have given $74 million so far this election cycle, according to the center's data.
Obama, who once pledged to raise $1 billion for his reelection effort, now claims the soaring spending of outside groups is leaving him at a disadvantage.
"I will be the first president in modern history to be outspent in his re-election campaign, if things continue as they have so far," Obama said in a recent fundraising e-mail to supporters.
The top Super PAC supporting Mitt Romney, the presumptive Republican nominee, has so far raised five times as much money as the Super PAC supporting Obama.
"We are seeing more money spent than any time in American history - a lot of it undisclosed, coming from folks who can write $10 million checks," Obama said at a recent campaign event.
Obama says the Citizens United decision enabled a very wealthy few to unfairly control the election narrative. Three-quarters of all outside spending comes from just 100 individual donors, according to campaign finance reports.
"But there are plenty of billionaires on the Democratic side as well," said Jeff Stein, a political analyst and communications professor at William Penn University in Iowa.
Obama's claims that outside groups are giving Republicans an unfair advantage is intended only to instill a sense of urgency in his own supporters, Stein said. In truth, Citizens United has leveled the playing field with unions, he said.
"Individuals previously did not have a way to participate in elections in the same way that unions could in using money from members, often when members did not have a choice," Stein said. "The impact of unions seems to be diminishing generally in society and this is just another manifestation of that."