There's an ongoing debate about exactly what role senators should have in the process of confirming judicial nominees. The Constitution gives the President the role of appointing people to certain positions, including "Judges of the Supreme Court", but this role is qualified. It is to be done "by and with the Advice and Consent of the Senate".
At this point there are two main views about what that advice and consent is supposed to be. Some senators have consistently maintained that ideology can play a role. If a senator disapproves of the ideology or perceived ideology of the nominee, it's perfectly fine to vote against the person's confirmation. Other senators have consistently maintained the view that senators should give significant deference to the president, voting to confirm any mainstream nominee who is qualified enough, even if the person tends significantly to the other side of the political spectrum or to a contrary judicial philosophy.
There are reasons for each view. Deference to the president makes some sense. When we vote for president, we do so while knowing what sort of judges the candidate is likely to appoint. Anyone who voted for Barack Obama while thinking he would appoint judicial conservatives to the bench is an idiot. Anyone who voted for George W. Bush while expecting him to appoint liberal justices to the Supreme Court wasn't paying attention to the kinds of justices he said he admired and would appoint. Mistakes can happen (as with Justice Souter with Bush's father), but you shouldn't expect your preferences to be fulfilled with judicial appointments if you vote for someone for president who has opposite preferences. Senators on the other side might say that elections have consequences and that presidents are owed some deference due to the political process. On the other hand, elections have consequences. Senators are elected. They represent the preferences of their constituents, and isn't the function of senators in judicial appointments part of what you should consider when you vote for someone for that office? So even on the democratic process argument, you might think it cuts both ways.
But other considerations have been offered against giving presidents a lot of deference on judicial nominations. One problem is that you still need to decide when to defer and when not to defer. How far outside the mainstream counts as sufficiently outside? Chief Justice Roberts and Justice Alito were both presented by Democratic senators as being outside the mainstream of conservative thought on evidence that's actually pretty similar to the evidence being used to argue that Judge Sotomayor is not outside the mainstream of judicial thought. It turns out, then, that this view isn't really a coherent, unified position. It's not about whether to give the president deference but about how much deference to give the president.
No one wants the Senate to rubber-stamp whoever the nominee is no matter what, so qualifications must matter, but it's not clear the ideological considerations are really separate from qualifications. Some would argue that an ideology that's very extreme actually disqualifies someone from being a good judge, because a good judge would interpret the law accurately and fairly, and extremist judges of certain sorts do not. But according to a judicial conservative, liberal jurisprudence then counts as a lack of qualifications. Any nominee who is ideologically liberal in terms of judicial philosophy is not a qualified nominee, and senators can vote against them on that basis and call it a matter of the nominee not being qualified.
So I'm no longer sure that the distinction between qualifications and ideology really explains much in terms of what senators should pay attention to, at least not in any way that will be agreed upon by a significant number of senators. Several other considerations also might favor looking to ideology. Some have argued that too much deference to the president leads to extremist judges on both sides, since presidents will get away with as much as they can if the Senate just defers.
On the other hand, the same problem comes up in the other direction if you don't defer to the president's nominees. Some of the most thoughtful and creative judges are on the extremes, and this isn't always a bad thing. I happen to think Justice Thomas' judicial philosophy has raised a lot of important questions that both sides' judicial thought has benefited from. He certainly hasn't been the deciding vote in many Supreme Court cases for most of his time on the Court, but his thought has raised a lot of fruitful avenues of discussion and led to a reconsideration of a significant amount of accepted but questionable assumptions about the state of the law in this country. Yet there's no way anyone anything like him would have a chance of being appointed to the Supreme Court unless they had almost no judicial or academic record, and that's a real loss for the Supreme Court. If any independent and unusual thinker gets Borked, out of a desire to restrict the Supreme Court to moderates, we end up with moderates making moderate decisions when in some cases maybe the mediating position is simply wrong. This argument isn't a left-right issue. It should apply equally to both sides, on the assumption that the left and the right alike would resist presidential nominees for ideological reasons.
What we've got right now is an unusual situation, though. Both sides do not treat nominees alike. At least in recent years (i.e. during the previous two presidents), Republicans have been more willing than Democrats to defer to the president on judicial nominees. That doesn't mean they haven't used obstructionist tactics or blocked nominees with technicalities of Senate procedure to avoid a full vote by the Senate, but it's pretty clear that only one party has tried to filibuster a Supreme Court nominee who was by most accounts in the mainstream, and it's pretty clear that one party has obstructed presidential nominees of the other party by avoiding even giving them hearings at a much higher rate than the other. That party is the Democratic party.
I think it's pretty clear at this point that I don't think there are strong arguments for an absolute obligation of senators to defer to the president, even if I've had inclinations in that direction. That means some matters might be strong enough considerations to overcome any inclination to give the president such deference. Is the imbalance in successful judicial nominations a strong enough reason to overcome an inclination to give deference to the president and start considering what you might otherwise have seen as bad ideology? It might. It's not clear to me exactly what I would do if I were in the Senate right now, but I mainly want to claim that one conclusion follows from all this. Those who in the past have argued for giving the president deference could consistently argue otherwise now, and it might not just be because a different party is in control of the White House. It doesn't have to be political posturing for someone to come to the conclusion that important judicial principles are too easily at stake for one party to consider ideology and the other not. So a senator who has previous argued for not considering ideology except in extreme cases might now consider ideology even in non-extreme cases, without necessarily being inconsistent with previous views. If the facts have changed, the same underlying principle could lead to a change in behavior.
I say this because I fully expect that Republicans who vote not to confirm Judge Sotomayor are going to be labeled inconsistent if they argued a few years ago that Judges Roberts and Alito should be confirmed. I don't think that charge can stick as easily as one might think. I'm not sure if I would vote against confirming her if I were a senator. I do see some things in her record that worry me. I certainly wouldn't advocate filibustering her, which the arguments then-Senator Obama gave about the Alito nomination should lead him as president to advocate senators doing. But I don't think any senator who votes against her is necessarily inconsistent even if it's a senator who has advocated deference to the president in the past.