But the list includes people involved in the business of influencing government.

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Michael Kempner, for example, is president and chief executive officer of MWW Group, a public relations firm with a large lobbying business. Kempner, who raised more than $500,000 for Obama's reelection bid, is not himself a registered lobbyist.

Also on Obama's list are two fundraisers linked to Solyndra LLC, the California solar company that received a $528 million Energy Department loan and then later went bankrupt, prompting a federal investigation.

Obama's re-election campaign also received about $200,000 in contributions bundled by family members of a Mexican casino owner who fled the U.S. after facing drug and fraud charges.

The Obama campaign said it has returned the money. More than 1,000 bundlers collectively raised $152.2 million for the 2008 campaigns of Obama and his opponent, Republican Sen. John McCain, according to the nonprofit Center for Responsive Politics.

Romney's Republican opponents also haven't identified their bundlers. But the stakes are higher for Romney, who holds a huge fundraising lead over his rivals.

Romney's campaign raised nearly $57 million in 2011 from individual contributions, which are capped by law at $2,500 each for primary and general elections.

Santorum, the former Pennsylvania senator who stunned Romney by sweeping contests last week in Minnesota, Missouri and Colorado, raised just $2.2 million last year.

Gingrich, the former House speaker, raised $12.6 million last year and Paul, a Texas congressman, collected $25.5 million.

Santorum, Gingrich and Paul have not reported having any registered lobbyists who are bundlers, according to the FEC records.

Money is the lubricant for presidential campaigns. But Santorum, seeking to maintain momentum from his recent victories, sought to use Romney's financial dominance against him with a David-versus-Goliath message.

"We're not going to win this race the way Gov. Romney has won the states he's won already, by outspending his opponent by 5-to-1 and beating them up. He's not going to outspend Barack Obama 5-to-1," Santorum said last week in Dallas. "How are you going to win an election if your greatest attribute is 'I'll spend more money than the other person?'"