In an unusual move, Senate Minority Leader Harry Reid (D-Nev.) will appear tonight in a 90-second television ad calling on Republicans to stand down on their threat to eliminate the filibuster for judicial nominees.

The ad will appear on the eve of the historic showdown over the minority party’s rights to block President Bush’s judicial nominees.

The commercial will be shown nationally on CNN, CNN Headline News and Fox News Channel and in the Washington, D.C., area on local television, according to Reid aides. The ad is scheduled to run between 7:50 and 8 p.m. and will be paid for by the Alliance for Justice, a liberal advocacy group that opposes the GOP’s proposal to change the judicial filibuster rule.

“Unfortunately, some Senate Republicans are trying to give President Bush power no president has ever had — the ability to personally hand out lifetime jobs to judges — including the Supreme Court, without consensus from the other party,” Reid says in the ad, according to a script provided to Roll Call. “This abuse of power is not what our founders intended. It’s wrong for one political party — be it Republicans today or Democrats tomorrow — to have total control over who sits on our high courts and rules on our most basic rights.”

Reid’s decision to appear in an advertisement during prime time illustrates the high stakes associated with judicial nominations. Eliminating the filibuster would allow Bush to nominate more conservative judges to the federal bench without worry that Democrats would filibuster them. This is especially significant given that there is the likelihood of at least one vacancy on the Supreme Court this summer.

So far, advocacy groups representing both sides of the issue have been running a combination of national cable ads and commercials targeting individual Senators in their home states.

“Sen. Reid is doing everything he can to stop the Republican abuse of power and preserve checks and balances,” said Rebecca Kirszner, a Reid spokeswoman. “He will take his case directly to the American people tonight and urge responsible Senate Republicans to stand up for a fair and independent judiciary.”

Even as party leaders prepared for a showdown vote, a dozen Senators from both political parties continued negotiations to try to avoid the so-call nuclear option. As of 1 p.m., no agreement had been announced.

Debate resumed on the nomination of Priscilla Owen to a seat on the 5th U.S. Circuit Court of Appeals with a “bed check” vote scheduled for 5:30 p.m.

In his opening remarks Monday, Majority Leader Bill Frist (R-Tenn.) said he hopes to have the vote occur late Tuesday morning.

While it was not certain if the Senate would stay in session overnight, Senate workers began the symbolic delivery of cots to the Capitol. In the very least, Frist said he plans to keep the floor open until every Senator has a chance to speak on the issue.

“In all likelihood it will be a very, very late night tonight,” Frist predicted.