Washington – Pennsylvania Sen. Pat Toomey held firm to his stand Wednesday that the next president, not President Obama, should fill the vacancy on the Supreme Court.

"With the U.S. Supreme Court's balance at stake, and with the presidential election fewer than eight months away, it is wise to give the American people a more direct voice," Toomey said shortly after Obama nominated Merrick Garland, chief judge of the U.S. Court of Appeals for the District of Columbia, to replace the late Justice Antonin Scalia.

Toomey said he would "be happy to carefully consider" Garland – if the judge is nominated again by the next president.

The incumbent senator is facing Democratic pressure to relent as he heads into an already difficult re-election climb this year. He is now part of a national fight on the high court.

Liberal groups have vowed to make Toomey's resistance a central piece of their bid to unseat Republicans this fall in key races in Pennsylvania and other moderate-to-blue states. They believe that now having a real face on the nomination will increase the pressure on the GOP.

"If Senate Republicans maintain this unprecedented level of obstruction and refuse to do their jobs, we'll make sure those Republican Senators who are up for reelection this year, like Sen. Toomey, are replaced with people who will," said Brad Woodhouse, president of Americans United for Change, one of the groups leading the push against Toomey and others.

One of Toomey's potential Democratic challengers, Katie McGinty, launched a television ad Wednesday afternoon slamming him for "obstructionism" over the nomination. Fellow Democratic candidates Joe Sestak and John Fetterman, the mayor of Braddock, have similarly attacked him.

Democrats are also pressing Republican Senators up for re-election in swing states such as Illinois, Ohio and New Hampshire.

Toomey's critics have already launched robocalls targeting him and staged events outside his offices. More protests are scheduled next week.

Democrats believe they have a potent issue they can use to either force Republicans to hold a vote, or pay a price in elections: Polls show that a majority of Americans, including independents, believe the Senate should act on an Obama nominee to replace Scalia.

Conservative groups, however, are rallying support for Republicans, calling for the GOP to deny Garland a hearing or confirmation vote. They fear that any Obama pick will tilt the court to the left for decades to come.

"The best thing for the nation is to trust the American people to weigh in on who should make a lifetime appointment that could reshape the Supreme Court for generations," said Carrie Severino, chief counsel of the conservative Judicial Crisis Network.

A Toomey campaign spokesman said McGinty and Sestak "would be a rubber stamp for any Obama nominee that would move the balance of the Supreme Court to the liberal side.  That's irresponsible."

Toomey has previously suggested that holding hearings or even meeting with a nominee would not make a difference, since his opposition hinges not the pick's credentials, but on concern over changing the court's direction so close to an election.

His office would not say Wednesday if that position has changed now that Garland has been chosen.

Democratic senators from the Philadelphia area pressed Republicans to "do your job" and offer Garland fair consideration after Obama's announcement.

"The American people expect us to do our job, without delay and politically-motivated excuses," said a release from Sen. Bob Menendez (D., N.J.).

Sen. Bob Casey (D., Pa.) called Garland "an eminently qualified nominee" and called for "a fair hearing and a timely vote."

The only local member of the Senate judiciary committee, Sen. Chris Coons (D., Del.), said the coming months, will "send a strong signal about whether our constitutional order can still function."

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