Sen. Ben Nelson is catching a lot of grief from within his party for being the first Democratic Senator in decades to oppose his president’s pick for the Supreme Court. But the Nebraskan is hearing none of it.

“Are they from Nebraska? Then I don’t care,” a defiant Nelson said Tuesday.

Nelson, who for months has broken with his party on a variety of high-profile issues, on Friday became the first Democratic Senator to oppose Elena Kagan’s Supreme Court nomination. Nelson has said Kagan is unfit to serve on the high court, noting her lack of judicial experience and his constituents’ concerns about the installment.

Kagan’s nomination, now being debated on the Senate floor, is all but a sure thing. With the exception of Nelson, every Democrat is expected to vote to confirm her. And GOP Sens. Lindsey Graham (S.C.), Olympia Snowe (Maine), Dick Lugar (Ind.), Susan Collins (Maine) and Judd Gregg (N.H.) have announced their support.

Nelson’s defection casts a shadow on a near-flawless confirmation performance for Kagan: Republicans now can claim that the solicitor general had “bipartisan” opposition.

Nelson seemed unfazed about that possibility on Tuesday, arguing that Kagan hasn’t demonstrated she is qualified to serve on the Supreme Court and taking issue with her position on the Second Amendment.

Nelson, who supported Sonia Sotomayor’s confirmation last year, argued that Kagan’s previous comments and political positions simply raise too many unanswered questions for him to support her. There are “enough comments made to raise doubts, [and] I’m not in the position to quell those fears. Including my own,” Nelson said.

Still, Nelson’s opposition has outraged the left flank of his party and kicked off speculation — which he has vehemently denied — that he may bolt the party next year if Republicans make significant gains in the midterm elections. Nelson is up for re-election in 2012.

Several Republicans were quick to point out that Nelson will be the first Democratic Senator to vote against his party’s Supreme Court nominee since 1968 when a bipartisan group of Senators blocked President Lyndon Johnson’s nomination of then-Justice Abe Fortas to become chief justice. Fortas was under an ethical cloud for accepting thousands of dollars from corporations for speaking engagements, and he was ultimately forced to step down from the court.

[IMGCAP(1)]Publicly, Democrats didn’t want to criticize Nelson for his promise of a “no” vote even though they quickly dismissed his line of reasoning. “I never suggest motives for Senators who can make up their own minds, but I do know we’ve confirmed a lot of people around here who are not nearly as qualified as she is,” Judiciary Chairman Patrick Leahy (D-Vt.) said.

Sen. Benjamin Cardin also declined to comment directly on Nelson’s decision, but he defended Kagan’s fitness for the high court.

“We have strong bipartisan validators on her qualifications. … I don’t think there’s any question” that she is qualified to serve, the Maryland Democrat said.

Privately, however, Democrats were less forgiving, complaining that with a potentially tough re-election looming in 2012, Nelson is trying to keep tight with the right. Nelson faced a firestorm of criticism during last year’s health care debate for securing a special Medicaid funding carve-out for his state, dubbed by critics as the “Cornhusker Kickback.” Nelson later repudiated the provision, which was ultimately removed.

“It’s a very transparent political move to cover his own tracks on a health care deal that he pushed at his own choosing,” a Senate Democratic aide said.

Those criticisms notwithstanding, Nelson has spent much of the year breaking with his fellow Democrats, and the Kagan vote is no exception. The Senate is poised to approve her installment by the end of the week.

Senators kicked off debate Tuesday morning with the predictable arguments.

Leahy stressed that Kagan is qualified for the lifetime appointment and said her confirmation hearings before his panel demonstrated that she would be an impartial, fair-minded justice.

“We do not need judges or justices to pass a litmus test from the right or the left. We need judges and justices who will respect the laws passed by Congress and appreciate that adherence to precedence is the foundation of public confidence in our courts. We need judges and justices who will fairly apply the law and use common sense, judges and justices who appreciate the proper role of the courts in our democracy,” Leahy said.

“It is a standard I believe Solicitor General Kagan has met. Solicitor General Kagan not only has the necessary qualifications to be a Supreme Court justice but has also demonstrated her respect for the rule of law, her appreciation for the separation of powers and her understanding of the meaning of our Constitution,” he added.

Judiciary ranking member Jeff Sessions (Ala.) used his opening floor statement to frame the GOP’s case against Kagan, taking her to task on a variety of issues including her judicial philosophy, handling of military recruiters while dean of Harvard Law School, work as solicitor general and views on abortion.

“She’s young, but her philosophy is not so recent. It is an old, bankrupt judicial activism, a philosophy the American people correctly reject,” Sessions argued.