The state GOP had argued that Gov. Deval Patrick - a Democrat - exceeded his constitutional authority by appointing Paul G. Kirk Jr. on Thursday. Lawmakers passed a bill this week giving Patrick the power to choose an interim senator, but laws usually take effect in 90 days.
Patrick got around that delay by signing a letter that declared the bill an emergency. The appointment is critical to his fellow Democrats, who need Kirk's vote to restore their 60-vote, filibuster-proof margin in the Senate. President Barack Obama and his staff lobbied for the change as they try to win approval this year for their top legislative priority, overhauling the nation's health care system.
Suffolk Superior Court Judge Thomas Connolly denied the party's request to delay Kirk's swearing in. Vice President Joe Biden was slated to do so at the U.S. Capitol two hours after the ruling was issued.
Connolly also granted the state's request to dismiss the case.
"The (Republican) Party has not shown that it has a chance to succeed on the merits and, therefore, any risk of harm to the party will not outweigh the risk of harm to the governor and the commonwealth," Connolly wrote in his decision.
The GOP did not immediately say if it would appeal.
On Wednesday, the Legislature amended the state's Senate succession law so the governor could appoint an interim senator during a five-month special election campaign to fill the seat permanently. Kennedy died Aug. 25 of brain cancer.
Legislative Republicans, as well as a host of Democrats, objected to the change, and after it was approved, they defeated a separate request to attach an emergency preamble to the bill. Patrick added the emergency preamble to the bill just moments before he named Kirk, the former chairman of the Democratic National Committee, to succeed Kennedy.
"The governor's power to declare an emergency is not absolute," attorney James O'Brien argued on behalf of the Republicans. "By granting the governor the power to appoint by way of an unconstitutional maneuver, this establishes a dangerous precedent."
Connolly sounded a skeptical note, foreshadowing his eventual decision, by asking O'Brien to define the irreparable harm the party would suffer - one requirement for granting an injunction.
"The net effect of the motion is that the commonwealth would not have two senators in Washington for 90 days?" Connolly asked O'Brien.
"That is correct," the attorney replied.
"How does that affect the Massachusetts Republican Party?" the judge asked.
O'Brien said the irreparable harm was the abuse of the constitutional process by allowing one person - the governor - to supersede the authority of the Legislature.
Patrick put the law into effect immediately by declaring it was subject to a citizen referendum. In that case, a governor is allowed to declare an emergency and make the law effective immediately. The intent of that constitutional provision is to prevent a group of angry citizens from pre-empting a law during the 90-day enactment period.
"We believe the irreparable harm is the misuse of the constitution," O'Brien told Connolly.
Assistant Attorney General Peter Sacks, representing the governor, said the GOP confused the referendum process. Patrick said the emergency the state faced was lack of representation during the health care and other debates under way in Washington.
"The urgency is not to escape the complaint," Sacks said. "There is simply no reason to delay it and every reason in the world, your honor, to make it as quick as possible."