WASHINGTON (Reuters) - The White House tried to defuse Republican charges of hypocrisy against President Barack Obama over Supreme Court nominees on Wednesday as liberal activists swung into action and the top Senate Democrat predicted a Republican “cave-in.” The White House turned up the heat on the Republican-led Senate to allow fair hearings and a timely vote on Obama’s impending selection to fill the court vacancy left by Saturday’s death of conservative Justice Antonin Scalia. The Democratic president’s nominee could change the balance of power on the top U.S. court - Scalia’s death left it with four conservative and four liberal justices - and a monumental fight is brewing over Obama’s pick for the lifetime appointment. Republicans, led by Senate Majority Leader Mitch McConnell, have said Scalia’s seat should remain vacant until Obama’s successor takes office next January so voters can have a say in the selection when they cast ballots in the Nov. 8 presidential election. The Senate must confirm any Supreme Court nominee. “I, first of all, think that they’re going to cave in,” Senate Democratic leader Harry Reid, speaking in Reno, Nevada, said of the Republicans. “I think the president’s going to give us a nominee that’s a good one, and I think they’re going to have to hold hearings and have a vote.”     Obama has argued the Senate has a constitutional duty to consider his nominee. White House spokesman Josh Earnest was put on the defensive over Obama’s actions a decade ago as a member of the Senate when he tried to block the nomination put forward by his predecessor in the White House, Republican George W. Bush, of conservative Samuel Alito to the nation’s highest court. As a first-term senator from Illinois, Obama used a procedural maneuver called a filibuster. Alito was confirmed anyway.      “Some Democrats engaged in a process of throwing sand in the gears of the confirmation process. And that’s an approach that the president regrets,” White House spokesman Josh Earnest told reporters.     Earnest portrayed Obama’s vote to try to block Alito as “symbolic” and sought to contrast it to “Republicans’ reflexive opposition” to Obama nominating a justice to replace Scalia. McConnell spokesman Don Stewart said the filibuster that Obama joined undercut the White House’s argument. Liberal groups including MoveOn.org and the Progressive Change Campaign Committee said in a conference call with reporters they would mount a campaign to prod Republican senators to allow hearings and a vote on Obama’s nominee. They said they already had gathered some 500,000 petition signatures opposing McConnell’s stance. Republican Senator Chuck Grassley, who chairs the Judiciary Committee that considers high court nominees, has not ruled out holding hearings, although he has offered mixed messages about how to proceed. “Grassroots voices are going to be the key to making Senator McConnell back off,” Senator Chuck Schumer, a member of the Senate Democratic leadership, told reporters. Earnest declined to rule out that Obama would make a recess appointment: naming someone to the job on a temporary basis while the Senate is on a recess, bypassing the confirmation process. The White House has been in touch with the offices of “multiple” senators of both parties about the court vacancy, Earnest said. During a speech at Yale Law School in Connecticut, liberal Justice Stephen Breyer called for a moment of silence to honor Scalia, telling the audience: “It’s going to be a grayer place without him.”  Earnest said Obama and first lady Michelle Obama would pay their respects to Scalia on Friday when the late justice’s body lies in repose at the Supreme Court building. Earnest said Obama would not go to Scalia’s funeral Mass on Saturday in Washington but that Vice President Joe Biden would attend. The remaining eight justices have canceled a meeting set for Friday to discuss action on future cases but are due next week to hear scheduled oral arguments in pending cases. 