Mitt Romney, favourite for the Republican presidential candidacy
"This is just the beginning of what we are going to see this election year," said Anthony Corrado, a campaign finance expert at Colby College in Maine, adding it was "striking" to see unions involved at such an early stage.
In the Citizens United case of 2010, the Supreme Court
upended campaign finance laws, ruling that unions and corporations had a right to "free speech" and allowing the creation of "super-Pacs" that can raise unlimited amounts of money.
Labour unions had not previously been allowed to co-ordinate directly with political groups such as Priorities USA and nor had they been able to spend from their general treasuries to finance ads.
AFSCME, the largest public sector union, spent USD1m on ads before the Florida primary, while the SEIU is building on its Florida campaign with a "
six figure buy" in Nevada ahead of Saturday's caucuses.
"This is part of our broader campaign of knocking on doors and talking to friends and family about core issues," said Brandon Davis, SEIU's national political director, describing SEIU as having a "99 per cent agenda".
"We want to begin an early conversation because the massive influx of resources that Mitt Romney has shown himself to have," he said. Campaign finance records released this week showed Mr Romney and his supporters spent USD17m on advertising in the early voting states.
Even though the Florida primary has passed, the SEIU is continuing to hit the airwaves in the Sunshine state, which will be a key battleground in the general election.
The SEIU was also the largest
donor to Priorities USA in the second half of 2011, giving it USD1m.
Unions have long been active in supporting Democrats but they have generally been focused on their own members and mobilising them to get out and vote.
This year, both the SEIU and AFSCME unions are funding attack ads to try to weaken Mr Romney, the candidate that Mr Obama's re-election campaign most fears, and make it more difficult for him to capture the Republican nomination. They have also given about USD2m to the House Majority super-Pac that aims to return Congress to Democratic control.
Meanwhile, AFL-CIO, the labour federation, has started a super-Pac, Workers' Voices, which raised USD 3.7m in the last six months of last year. Like other super-Pacs, Workers' Voices can raise unlimited funds from corporations, individuals and other unions.
But it will focus exclusively on mobilising voters and will not be funding advertising, said Jeff Hauser of the AFL-CIO. "Our traditional approach has been quite effective, with union members talking to
fellow union members," Mr Hauser said. "With a super-Pac, our members will be able to talk not only to other union members but to other working people and their neighbours."
The advent of union attack ads and super-Pacs highlights one of the most surprising effects of the Citizens United ruling. Opponents had warned of corporations spending huge amounts of money to influence elections, but in fact it is unions that have taken advantage of the changes so far.
Still, the amount they are spending is pennies compared with the tens of millions being spent by the super-Pacs that support Mr Romney and Mr Gingrich - for now. "Once President Obama has a challenger and we get closer to the conventions and the election, that will start to change," said Bill Allison of the Sunlight Foundation, which monitors campaign donations.