WASHINGTON  The Supreme Court has handed lobbyists a new weapon. A lobbyist can now tell any elected official: if you vote wrong, my company, labor union or interest group will spend unlimited sums explicitly advertising against your re-election.

“We have got a million we can spend advertising for you or against you  whichever one you want,’ ” a lobbyist can tell lawmakers, said Lawrence M. Noble, a lawyer at Skadden Arps in Washington and former general counsel of the Federal Election Commission.

The decision seeks to let voters choose for themselves among a multitude of voices and ideas when they go to the polls, but it will also increase the power of organized interest groups at the expense of candidates and political parties.

It is expected to unleash a torrent of attack advertisements from outside groups aiming to sway voters, without any candidate having to take the criticism for dirty campaigning. The biggest beneficiaries might be well-placed incumbents whose favor companies and interests groups are eager to court. It could also have a big impact on state and local governments, where a few million dollars can have more influence on elections.