There's actually a more complicated explanation

Than the one given by the union:

"There has been a coordinated campaign for the last 30 years to undermine the American middle class by weakening the power of workers to collectively bargain to raise their wages," said Mary Kay Henry, president of the Service Employees International Union, which represents nearly 2 million workers, many of them public employees.

She contends that corporate America, in an effort to keep middle-class wages in check, doled out hundreds of millions of dollars to support Republicans running for statewide offices.

And CNN, in its investigative reporting best, merely notes that "organized business interests" spent $828 million on the last election cycle at the state level, while organized labor spent a whole lot less.  The impression, of course, is that these businesses all give their money to Republicans and that's why they won and boo hoo hoo it's not fair. The union spin is that Republicans get all this business money in order to drive down your paycheck. It's a deceptively simple story, with an emphasis on 'deceptively.'

It's just not that simple, and it can be shown to be not that simple by actually looking at who gives to whom. The Republicans for their part really are simple: very few union dollars go to Republicans - on the order of 10% - ergo the vast majority of their money came from 'business interests.'  But what of the Dems? Where did their money come from? It's easy to say "unions" until we look at the numbers.

To pick a totally random* example, Cindy Neighbor is a good Kansas Democrat**. Whence came her campaign funds? Well according to CNN's source, other than the 25 grand of her own money she spent, these are the First Tier ($1000) donors:
Kansas Dental Association
Kansas Medical Society
Kansas NEA
Kansas Optometric Association
Kansas Contractors Association
Kansas Hospital Association
Johnson County Democrats
A T & T

I only see 1 union in that group of 8, and 1 party donation. You can follow the money down and you'll find more unions and you'll find trial lawyers and other reptilia, but you'll also find companies like Cox Communications and Southern Star Central Gas Pipeline company.

But before we ask why this is, I want to take a look at one other oddity, If you click on the Kansas Dental Association, it will become quickly apparent that they gave money to a lot of people running for office, 151 of them in total.  Since there were only 110 offices up for election, it's obvious they were playing both sides of the fence***. In fact, their top gifts went mostly to Republicans, but there were a few Dems like Cindy Neighbor and Dennis McKinney in there as well, and a whole lot of Dems in the lower tiers.

Why were those Democrats in there? It's not because they like teeth more than their Republican opponents, but because they were in large part incumbents.  Remember back when the Dems ran everything in Washington? They were able to outraise the GOP. Now that the GOP has the house, they are able to outraise the Dems. This difference is not in union money - which always goes to Democrats - it is money from 'organized business interests.' This leads us to an obvious conclusion that unions are embarrassed by and CNN is too busy covering Justin Bieber's new haircut to be bothered with: Unions and Businesses give for different reasons. 

Unions give money to Democrats because they are bosom buddies, both ideologically and functionally: the Dems favor laws that favor unions, whereas the unions direct-deposit worker and taxpayer money into Democratic coffers. On the other hand, business interests give to incumbents because they need to buy access to government, whether it is run by Democrats or Republicans****. The Dental Association gives Cindy $1000 because it wants to be able to talk to her when she gets re-elected (she didn't, alas), not because it wants to drive down the wages of those nice young ladies who clean your teeth twice a year.

* random in the sense that I didn't know what the numbers were going to look like before I picked her and I have no reason to suspect that she differs from other Dems.
** She actually used to be a Republican, before the lovely and gracious Rogue stole her precinct seat, so there's a bit of history there. In fact, many Kansas Democrats of note, including our last governor and penultimate attorney general, used to be Republicans.
*** In fairness, they also gave to people who were not running, like state senators.
**** This is not to say that businesses don't prefer Republicans; of course they do, especially the Washington kind that are indistinguishable from Democrats behind closed doors.