A leading watchdog group has filed complaints with the top US campaign finance regulator against three top Republicans – Jeb Bush, Scott Walker and Rick Santorum – and Democrat Martin O'Malley, claiming they have already violated federal law by contemplating presidential bids without formally "testing the waters".

The complaints follow a report by the Guardian on Monday detailing that many potential presidential candidates are walking a careful line as they consider bids for the White House without taking any specific action that would trigger scrutiny from the Federal Election Commission (FEC).

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According to the Campaign Legal Center, even those actions – traveling from state-to-state, raising money and establishing field offices as non-candidates – "appear to be violating federal law".

"Publicly denying that they are candidates does not exempt these presidential hopefuls from federal election laws passed by Congress to keep the White House off the auction block," said Paul S Ryan, the watchdog's senior counsel.