Colorado Republicans will sue to overturn voter-approved state limits on some campaign contributions after a U.S. Supreme Court ruling Thursday that tossed out restrictions on corporate involvement in federal races.

“Our firm will be bringing a challenge to this law in the coming days,” said Ryan Call, an attorney with Hale-Friesen.

Call said the firm will represent the Colorado Republican Party in the suit and is trying to put together a cross section of plaintiffs.

“It will be a broad coalition,” he said. “It will be partisan groups. It will be trade associations, individual companies and corporations.”

The Supreme Court threw federal campaign finance into turmoil Thursday with a broad rejection of parts of a 63-year-old law that prohibited companies and unions from using revenues to run campaigns against or for political candidates.

In 2002, Colorado voters approved a ballot measure that banned direct corporate or union expenditures in state races. Given the Supreme Court’s action Thursday, Colorado Common Cause director Jenny Flanagan said a challenge of the state law was almost inevitable.

“I think we’re very vulnerable given the breadth of the court’s decision today on the corporate issues,” she said. “I absolutely expect it.”

Common Cause was one of the major sponsors of the 2002 ballot measure.

Call said the lawsuit probably will also challenge other parts of Colorado’s law, such as the $400 limits on donations to state legislative candidates by individuals and political action committees and the creation of small-donor committees that are allowed to contribute 10 times the limit because they get their money from donations of $50 or less.

Small-donor committees have been commonly used by labor unions, who use money from union payroll deductions, to make large donations to legislative races.

But Flanagan said overturning the limits and small-donor groups in Colorado would not immediately flow from the Supreme Court’s decision.

“It did not address these other issues,” she said. “It’s not clear in those areas.”

Colorado Secretary of State Bernie Buescher said he will work with state Attorney General John Suthers to see whether the Colorado finance laws need to be changed by the legislature to conform to the court decision.

The looming lawsuit is one of several aftershocks expected from what politicians and analysts said is a “landmark” decision affecting how political campaigns will unfold, starting this year.

It could lead to an autumn barrage of political ads and mailings directly financed by corporations and unions — and out of candidates’ control.

The decision allows corporations and unions to spend unlimited amounts of money from their treasuries to support or oppose federal candidates all the way through Election Day.

It’s likely to be a factor in Colorado’s U.S. Senate race, candidates said.

“The problem with this decision is it gives each candidate less control over their own race and gives outside groups more control,” said Craig Hughes, campaign manager for Democratic Sen. Michael Bennet.

Cinamon Watson, spokesperson for Republican U.S. Senate candidate Jane Norton, said the campaign expects to see more money from outside interests this year.

“It’s going to be a fight to the finish,” Watson said.

Former state House Speaker Andrew Romanoff, who is challenging Bennet for the Democratic nomination, said the ruling “opened the floodgates for billions more” in special-interest money.

But Mark Grueskin, a leading election-law specialist at Isaacson Rosenbaum, said it remains to be seen if the decision leads to a torrent of corporate and labor spending in political races here.

The economy is down, attention will be focused on three ballot measures to limit state government spending, and corporations and interest groups will need to be careful about the political messages they attach their names to, Grueskin said.

“It’s a risk-laden strategy,” he said.

Burt Hubbard: 303-954-5107 or bhubbard@denverpost.com