Senate Majority Leader Mitch McConnell,  center, walks with Sen. Pat Toomey  of Pennsylvania to the Senate chamber at the Capitol on Feb. 10. Both have been targeted by political activists hoping to influence the Senate’s handling of a potential Supreme Court nominee. (J. Scott Applewhite/AP)

A coalition of liberal activist groups say they will deliver at least a half-million petition signatures Friday urging the swift consideration of President Obama’s promised Supreme Court nominee to the doorstep of Senate Majority Leader Mitch McConnell (R-Ky.), who has said that any confirmation should wait until after the inauguration of a new president.

The groups promising to deliver the petitions to McConnell’s Louisville office include the Progressive Change Campaign Committee, MoveOn.org Civic Action, Color of Change and several other organizations that are pressing Senate Republicans to fill the vacancy created by Associate Justice Antonin Scalia’s death Saturday at 79. The delivery of the petitions is one of the most dramatic gestures yet from activists on both sides of the political divide who have mobilized to feverish levels in recent days, citing the sky-high stakes of a nomination battle.

“Grassroots pressure will be the key to winning this fight,” said Adam Green, the PCCC’s co-founder, in a conference call with reporters Wednesday in which he cited “nearly unprecedented grassroots energy and public outcry” over Republican plans to block an Obama nomination. McConnell said Saturday that the vacancy “should not be filled until we have a new president.”

The liberal activists announced their plans targeting McConnell on Thursday, the same day that a conservative legal group, the Judicial Crisis Network, announced a seven-figure ad campaign aimed at persuading the majority leader and several other GOP senators to maintain their opposition to any Obama nominee.

[Ads pressure Republicans not to cave on SCOTUS vacancy]

“We want to thank the U.S. senators who say that the American people should decide who picks the next Supreme Court justice,” said the group’s chief counsel, Carrie Severino, in a statement. “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 ads are set to run during Sunday’s morning talk shows in the home states of McConnell, as well as GOP senators Kelly Ayotte (N.H.), Chuck Grassley (Iowa), Ron Johnson (Wis.), John McCain (Ariz.), Rob Portman (Ohio) and Pat Toomey (Pa.). Ayotte, Johnson, Portman and Toomey are amid tough re-election campaigns. McCain is a Republican elder statesman whose opinion on the nomination could be influential, while 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 robocall 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 a request for donations Tuesday signed by Sen. Barbara Boxer (D-Calif.), calling McConnell’s refusal to consider an Obama nominee “another outrageous example of the GOP putting partisan politics ahead of what is best for our country.”

The National Republican Senatorial Committee, meanwhile, sent out an appeal signed by McConnell himself saying, “We are at a crossroads in American history.”

“The American people deserve to have a say in the selection of a new justice to the Supreme Court,” he continued. “I hope you will stand with Senate Republicans by signing the petition and standing with us on principle in this historically challenging time.”