Super PACs have dominated spending in the run-up to the 2016 US presidential election, flooding the airwaves with advertisements costing millions of dollars.

But despite deep pockets, these independent political action committees that raise and spend money to assist or defeat candidates may not have the effect on voters some might have expected, said experts.

As of December, Super PACs, which cannot coordinate their efforts with a candidate's campaign, have raised more than US$320 million (S$453 million) and spent more than US$99 million in the 2016 presidential election cycle, according to the think-tank Centre for Responsive Politics.

More than half - US$52.2 million - of the money spent has gone to Republican presidential advertisements this year. This is up from just US$14.9 million spent on advertisements at this point of the election cycle in 2011, according to research group Wesleyan Media Project.

In 2010, two court rulings - Citizens United and SpeechNow - made it possible for independent groups to collect and spend unlimited contributions from individuals, corporations and unions, thus allowing for large contributions from wealthy donors. In contrast, individuals can contribute only up to US$2,700 directly to a presidential candidate's campaign.

But while the money spent by Super PACs is staggering, experts say it may not always translate into voter support.

University of Connecticut's political science professor Paul Herrnson said that before 2010, to be successful, a candidate would have had to raise money through many individual donations, fund the campaign, rise in the polls and continue raising money through supporters.

"But with Super PACs, it doesn't work that way. Scott Walker and Rick Perry had money but no popularity," said Prof Herrnson. "Jeb Bush leads in SuperPAC fund-raising but again, no popularity."

Mr Bush's Right to Rise Super PAC has raised more than US$103 million, which overshadows the US$24.8 million his campaign has raised as of Sept 30.

"It is true that Right to Rise's spending on the Jeb Bush candidacy has not lifted him in the polls," said associate law professor Ciara Torres-Spelliscy, a fellow at the Brennan Centre for Justice think- tank. "But part of the phenomenon is explained by the nearly endless free media coverage that the Republican front-runner in the polls (Donald Trump) has been given."

That said, "Super PACs do fill a void caused by the lack of resources", said Prof Herrnson, and can extend a candidate's longevity in the race. For example, when the Bush campaign had to lay off staff and cut costs, Right to Rise stepped in to run a "shadow campaign".

While candidates' campaigns are not allowed to coordinate with Super PACs, there are many ways to circumvent the rules. "Many Super PACs are headed by former aides who have a sense of how the campaign will run," said Prof Herrnson.

Also some campaigns put information up on micro websites which the public - and Super PACs - can access, if they know where to look. "It is uncoordinated, but there is a harmonising of efforts," he said.

In November, the Federal Election Committee, which regulates campaign finance, issued an advisory opinion allowing small group meetings between a candidate or official from a campaign, one Super PAC operative and one donor.

This would "likely make coordination between political campaigns and outside spenders even easier", said Prof Torres-Spelliscy, adding that it further erodes the already porous coordination rules.

There are candidates, however, such as real estate mogul Donald Trump and Vermont Senator Bernie Sanders, who are running campaigns without Super PACs.

Running the most traditional campaign is Mr Sanders, whose high number of small donors, experts say, could put him in better stead than a candidate with overflowing Super PAC coffers.

"A billionaire can give lots of money, but can only vote once. Thus millions of small donors may be far more important in a democracy than a few wealthy donors," said Prof Torres-Spelliscy.