Washington ? If you are John Cornyn, the 36 hours from last Tuesday night through Thursday morning had to be the best time of your life.

Cornyn, the junior senator from Texas, is the current chairman of the National Republican Senatorial Committee, the campaign arm of the Senate GOP.

Until Tuesday, he was the spokesman for an embattled minority, staring up at a 60-vote behemoth of 58 Democrats and two independents, fond of flexing its supermajority muscles and driving the opposition into the ground.

On Tuesday night, when underdog Scott Brown beat Martha Coakley in the Massachusetts special election to fill Ted Kennedy’s seat, Cornyn and his boss, Minority Leader Mitch McConnell, suddenly had the power — with 41 votes — to filibuster and defeat any legislation on which they were united.

They also acquired a convincing argument to use on campaign workers, candidates and contributors all across the country: If a backbench state senator can beat the Democratic machine in Kennedy-worshiping Massachusetts, cracking a lineup where no Republican had won in years, then 2010 is a year of opportunity for the GOP you don’t want to miss.

Republican campaign aides said prospective candidates and previously reluctant donors quickly signaled their interest in playing. And then serendipitously, on Thursday morning, the Supreme Court’s conservative majority delivered a decision that may well be the best news Republicans have received since the 2000 ruling in Bush v. Gore.

The holding in Citizens United v. Federal Election Commission, allowing corporations to spend unlimited corporate funds on their own election messages, means that the 2010 campaign will likely see a vast infusion of business-backed independent expenditures. The likelihood is that the main beneficiaries will be the Republicans recruited by Cornyn and, over in the House, by Rep. Pete Sessions of Texas, the chairman of the National Republican Congressional Committee.

The same Supreme Court decision sets a precedent for freeing labor unions to tap their treasuries for the funds that, in most cases, will find their way to Democratic candidates. But the potential resources are by no means equal, and an astute fundraiser such as Cornyn is likely to come out far ahead of his Democratic counterpart, Sen. Bob Menendez of New Jersey, in the race for resources.

But of course the Democrats have in President Obama a resource the Republicans can’t match. Obama broke all previous fundraising records in his 2008 campaign, with a combination of big-dollar events and Internet-fueled direct mail contributions.

By rejecting public financing of his post-convention campaign and relying instead on private contributions, Obama weakened the system of taxpayer-financed campaigns his predecessors (and opponent John McCain) had relied on.

This did not inhibit Obama from complaining that “with its ruling today, the Supreme Court has given a green light to a new stampede of special-interest money in our politics.”

Cornyn, playing it low-key as usual, simply said, “I am pleased that the Supreme Court has acted to protect the Constitution’s First Amendment rights of free speech and association.”

As a First Amendment fan myself, I had hoped the Supreme Court would find a way to safeguard conservatives’ right to make and distribute with corporate funds a propaganda film decrying Hillary Clinton’s candidacy for president. I think their voice should be heard.

But the five-member majority went much further. On its own, it extended itself far beyond what was necessary and knocked out well-established precedents in order to grant free rein for corporations to spend on politics as if they were citizens with a guaranteed voice in the election.

I agreed with the dissenting opinion of Justice John Paul Stevens and his three liberal colleagues, who said the majority opinion rejects “the common sense of the American people, who have recognized a need to prevent corporations from undermining self-government since the founding, and who have fought against the distinctive corrupting potential of corporate electioneering since the days of Theodore Roosevelt. It is a strange time to repudiate that common sense.”