The Senate's top Republican said Supreme Court nominee Sonia Sotomayor fails the test of impartiality despite her impressive background, while the senior Democrat defended her as a mainstream moderate, as the Senate prepared to debate the judge expected to be confirmed as the first Hispanic and third female justice.

Republican Sen. Mitch McConnell of Kentucky and Democratic Sen. Harry Reid of Nevada previewed their parties' dueling arguments Tuesday over President Barack Obama's nominee, who is expected to sail through the Senate this week with bipartisan support. Most GOP senators have lined up against Sotomayor, but a handful are siding with Democrats to support her.

"She's developed a 17-year record as a moderate, mainstream judge," said Reid, the majority leader. "I'm disappointed not more of my colleagues on the other side of the aisle are likely to vote for this outstanding nominee."

The two party leaders spoke on the Senate floor a few hours before formal debate was to begin.

Sotomayor, 55, is the daughter of Puerto Rican parents who was raised in a South Bronx housing project and educated in the Ivy League before going on to success in the legal profession and then the federal bench. Obama chose her to replace retiring Justice David Souter, a liberal named by a Republican president, and she's not expected to alter the court's ideological balance.

Still, Republicans call her an activist who would bring bias to the high court, pointing to a few rulings in which they argue she showed disregard for gun rights, property rights and job discrimination claims by white employees. They're also unsatisfied with Sotomayor's explanation of a 2001 speech — similar to comments she's made throughout her career — in which she said she hoped a "wise Latina" would usually make better decisions than a white male.

"Judge Sotomayor is certainly a fine person with an impressive story and a distinguished background. But a judge must be able to check his or her personal or political agenda at the courtroom door and do justice evenhandedly... It's a test Judge Sotomayor does not pass," said McConnell, R-Ky., the minority leader.

Democrats point instead to a long record of rulings in which Sotomayor has reached the same conclusions as judges who are considered more conservative. They call her a moderate who is restrained in her legal interpretations and argue that her controversial remarks — while perhaps worded inartfully — show nothing more than a belief that diverse experiences help a judge see all sides of a case.

Some in the GOP have faced a tough call about how to vote on Obama's nominee, torn between an impulse to please their conservative base by opposing her and a fear that doing so could alienate Hispanic voters. The vast majority are lining up with their core supporters against Sotomayor, despite her near certainty of being confirmed.

The decision was further complicated for some senators in both parties after the National Rifle Association announced it would downgrade senators who supported Sotomayor in its annual candidate ratings. Still, many Democrats and a couple of Republicans with perfect or near-perfect scores from the group are defying the NRA and voting for her anyway.