By Ross Colvin

WASHINGTON (Reuters) - President Barack Obama launched an extraordinary attack on the Supreme Court on Saturday, saying a ruling on corporate donations to political campaigns this week "strikes at democracy itself."

Obama's broadside was triggered by a 5-4 ruling by the court's justices on Thursday that removed long-standing campaign finance limits and allowed corporations to spend freely in campaigns for president and Congress. In the ruling, the court's conservative majority said the limits had violated corporations' constitutional right to free speech.

The ruling is expected to unleash a flood of money into this year's congressional elections. Obama's fellow Democrats face a struggle to retain control of the Congress amid voter unhappiness over double-digit unemployment, a record deficit, political gridlock in Washington and other matters.

"This ruling opens the floodgates for an unlimited amount of special interest money into our democracy," Obama said in his weekly radio and Internet address.

"It gives the special interest lobbyists new leverage to spend millions on advertising to persuade elected officials to vote their way -- or to punish those who don't. This ruling strikes at democracy itself," Obama said.

In the 2008 election cycle, nearly $6 billion was spent on all federal election campaigns, including more than $1 billion from corporate political action committees, trade associations, executives and lobbyists.

REPUBLICANS PRAISE RULING

Democrats fear that a lot of the corporate money unleashed into U.S. political campaigns by the ruling will go to Republicans, traditional allies of big business.

Republicans already were expressing optimism about erasing the Democratic majorities in the House of Representatives and the Senate in November's elections. Republican control of either the House or Senate could cripple Obama's ability to get key policy initiatives through Congress.

Republican Party chairman Michael Steele has praised the court's ruling, saying free speech strengthened democracy.

Obama blasted the ruling the day it was issued, and expanded on his criticism in Saturday's remarks.

The Supreme Court ruling dealt a powerful blow to efforts to rein in corporate influence and could even allow foreign corporations to "get into the act," Obama said.

"It will make it more difficult to pass common-sense laws to promote energy independence because even foreign entities would be allowed to mix in our elections," Obama said.

It also will impede efforts to pass financial regulatory reforms and will give the health insurance industry "even more leverage to fend off reforms that would protect patients," he said, referring to the industry's opposition to his efforts to overhaul the $2.5 trillion U.S. healthcare industry.

Obama came into office vowing to curb the influence of powerful lobbyists for banks, health insurance companies and other special interest groups in Washington.

He said his administration had barred gifts from lobbyists to administration officials and "closed the revolving door between lobbying firms and the government" so that no one in his administration would make decisions based on the interests of former or future employers.

Obama said he had instructed his administration to work immediately with Congress to develop a forceful, bipartisan response to the ruling.

"We have begun that work, and it will be a priority for us until we repair the damage that has been done," he said.