The bench chair of late Supreme Court Justice Antonin Scalia is seen draped in black wool crepe, a tradition dating back to 1873, at the Supreme Court in Washington. (Michael Reynolds/European Pressphoto Agency)

Activist groups across the political spectrum laid down battle lines Thursday for the coming fight over President Obama’s promised Supreme Court nominee, while the Senate leaders at the center of that fight bickered over positions they had taken years or even decades in the past.

After Republicans spent much of the week seizing on a 2007 speech by Sen. Charles E. Schumer (D-N.Y.) in which he recommended blocking any Supreme Court nomination made in President George W. Bush’s final 18 months in office, Democrats on Thursday circulated a 1970 law review article penned by Senate Majority Leader Mitch McConnell (R-Ky.) in which he suggested that “political matters” were not relevant to the Senate’s consideration of a nominee.

In a Washington Post op-ed published online Thursday evening, McConnell and Judiciary Committee Chairman Sen. Charles E. Grassley (R-Iowa) pointed to a 2005 remark by Sen. Harry M. Reid (D-Nev.), now the Senate minority leader. During the consideration of a Bush appeals court nominee, Reid said, “Nowhere in that document does it say the Senate has a duty to give presidential nominees a vote.”

McConnell, in a statement made shortly after the death of Justice Antonin Scalia was announced Saturday, called for the vacancy to be filled by the next president. Reid and Schumer, the No. 3 Democratic leader, are calling for swift consideration of Obama’s promised nominee.

The pliability of lawmakers’ positions reflects the sky-high political stakes of the nomination battle, in which the ability of Obama or another Democratic president to appoint a justice more liberal than Scalia could realign the court for years.

Those stakes have been reflected in the feverish activities of advocacy groups. In one of the more dramatic gestures, a coalition of liberal activist groups said it planned to deliver at least a half-million petition signatures Friday to McConnell’s office doorstep in Louisville.

“Grass-roots pressure will be the key to winning this fight,” said Adam Green, co-founder of the Progressive Change Campaign Committee, one of the groups behind the petition drive, in a conference call with reporters Wednesday. He cited “nearly unprecedented grass-roots energy and public outcry” over Republican plans to block any nomination.

Also on Thursday, a conservative legal group, the Judicial Crisis Network, announced a seven-figure ad campaign aimed at persuading the majority leader and several other Republican senators to maintain their opposition to any Obama nominee.

“The American people are fed up with Washington politicians, and the selection of the next justice is simply too important to leave to politics as usual,” the group’s chief counsel, Carrie Severino, said in a statement.

The ads are set to run during Sunday’s morning talk shows in the home states of McConnell as well as Republican Sens. Kelly Ayotte (N.H.), Grassley, Ron Johnson (Wis.), John McCain (Ariz.), Rob Portman (Ohio) and Patrick J. Toomey (Pa.).

Ayotte, Johnson, Portman and Toomey are amid tough reelection campaigns; McCain is a Republican elder statesman whose opinion on the nomination could be influential; and Grassley will decide whether to take up any Obama court nomination in his role as Judiciary Committee chairman.

Several of those senators are being targeted by liberals as well. For instance, People for the American Way, a left-wing advocacy group, sent a robo-call to Wisconsin activists featuring actor Martin Sheen asking them to contact Johnson, who said in a Tuesday radio interview he wants to “let the American people decide the direction of this country” through the presidential election.

“Republicans are playing politics with our Constitution and with the Supreme Court,” Sheen says in the call. “Sen. Ron Johnson has said he doesn’t think that Congress should even give a fair hearing to anyone nominated by President Obama. That’s irresponsible, and it puts partisanship above the law.”

Meanwhile, party campaign committees are trading fundraising appeals based on the court vacancy. The Democratic Senatorial Campaign Committee sent out the latest in several court-themed requests for donations Thursday, calling McConnell’s refusal to consider an Obama nominee “beyond irresponsible.”

The National Republican Senatorial Committee, meanwhile, sent out an appeal signed by McConnell himself: “The American people deserve to have a say in the selection of a new justice to the Supreme Court. I hope you will stand with Senate Republicans by signing the petition and standing with us on principle in this historically challenging time.”

Elise Viebeck contributed to this report.