Their decision is a mistake -- and worse, it's a breach of their duty.
Certainly, senators can vote for a Supreme Court justice who doesn't share their political point of view (if I had been a senator, I would have voted to confirm Justice Ginsburg, even though I disagree with her). Senators can vote for a Supreme Court justice whose approach to adjudication they believe to be wrong (again, if a senator, I would have voted for Justice Breyer, though I find his willingness to rely on foreign law repugnant). The Democrats won the election, and elections have consequences.
But it is wrong -- and a violation of one's duty as a senator -- to vote for someone who has indicated that she is either unwilling or unable to uphold her judicial oath. The oath soon-to-be-Justice Sotomayor will take is as follows:
"I, [NAME], do solemnly swear (or affirm) that I will administer justice without respect to persons, and do equal right to the poor and to the rich, and that I will faithfully and impartially discharge and perform all the duties incumbent upon me as a Justice of the Supreme Court under the Constitution and laws of the United States. So help me God."
It's not just Judge Sotomayor's repeated assertions about a "wise Latina woman" making better decisions than a white man. It's that she accepts -- and even embraces -- the notion of subjectivity in judging, as in this statement:
I simply do not know exactly what that difference [based on my personal experiences] will be in my judging. But I accept there will be some based on my gender and my Latina heritage.
Anyone who is not willing at least to strive for complete objectivity and impartiality in judging has no business on any court -- and certainly not on the Supreme Court. Yet, in Sotomayor's view, sometimes biases and prejudices are perfectly acceptable, as this statement indicates:
I willingly accept that we who judge must not deny the differences resulting from experience and heritage but attempt, as the Supreme Court suggests, continuously to judge when those opinions, sympathies and prejudices are appropriate.
Had the once-all-white-male judiciary of pre-civil-rights-era America been so comfortable with the notion that sometimes "opinions, sympathies and prejudices are appropriate," the struggle for equal rights would have been even more difficult. And if the entire judiciary now embraced Judge Sotomayor's vision, litigation would devolve into a system where parties would simply hope that they were of the "right" ethnicity and gender to engage the "sympathies and prejudices" of the judge charged with rendering a decision.
Her vision is a complete abrogation of the judicial role. No senator can vote, in good conscience, to confirm a judge who is so careless about the concept of "equal justice under law" -- even if they think they like the outcomes that Judge Sotomayor's "opinions, sympathies and prejudices" will precipitate.