At first glance, with Democrats a hair away from a filibuster-proof majority in the Senate, one would expect President Obama to have no trouble hand-picking a replacement for retiring Supreme Court Justice David Souter. 

But in an ironic twist, Pennsylvania Sen. Arlen Specter's switch to the Democratic Party this week could give Republicans on the Senate Judiciary Committee the upper hand in rejecting a nominee they find unacceptable.

That's because the Judiciary Committee, where Specter was the ranking minority member, requires the consent of at least one Republican to end debate and move a nominee to the full Senate for a vote.

"I think, in narrow terms, it could present a procedural problem at the committee level, unless the Democrats are going to change the rules of the committee midstream," William Jacobson, a professor of law at Cornell University, told FOXNews.com.

"Most people presume in a controversial nomination that Arlen Specter would have been the one most likely to vote with Democrats, since he prides himself on being independent of Republicans. But now that he moves over to the Democratic side, the president and Democrats lost their most likely minority vote."

A committee aide to Sen. Patrick Leahy, chairman of the panel, declined to comment on anything connected to Souter's expected retirement or a Supreme Court nomination.

Before the Republicans on the Judiciary Committee can form an opposition strategy, they will have to elect a new committee party leader. Sen. Orrin Hatch, R-Utah, has served as chairman before and would need a waiver from members to serve again. Next in line would be Iowa Sen. Charles Grassley.

Jacobson believes the most likely Republican to help Democrats on the committee is South Carolina Sen. Lindsey Graham, who was one of the Gang of 14, a group of seven Democrats and seven Republicans who averted a showdown on President Bush's judicial nominees in 2005.

"If Obama were to nominate someone clearly viewed as a political appointee ... then I think Lindsey Graham would be subject to pressure," Jacobson said. "On the other hand, if he were to nominate someone Republicans don't like but is qualified, like (Solicitor General) Elena Kagan, would Lindsey Graham feel compelled to go along with the gang of 14? I think that is something that remains to be seen."

Graham's office did not respond to a request for comment.

Democrats aren't powerless to stop a potential filibuster at the committee level. They could change the rules to allow the committee to vote on the nominee and send a recommendation to the full Senate without Republican consent.

But Jacobson believes that's unlikely to happen.

"The senators, as political as they can be, they have tended to put value on the rules of conduct," he said. "To change the rules to get a particular nominee confirmed would set a dangerous precedent. I doubt Democrats would want to do that."

He added that changing the rules might tick off the unpredictable Specter who has developed a strong respect for tradition.

"If Democrats were to change the rule to force through a nominee, he might vote with Republicans," Jacobson said.