So what? You're aware of the Democrats position in the US Senate? He's either doing this as part of a strategy to make Republicans look bad, or he honestly thinks it's better to fill it with a less ideological center-right individual before any conservative has a chance to fill it with another Scalia. If he truly thought that, then that wouldn't make him a 'Republican', that would make him (at least in my eyes), a Democrat/liberal making a bad decision.
Look at his record as a whole and judge him on that, not based on some action which almost certainly is not being done because he truly wants Sandoval. His past 2 picks show that. Things aren't as simple as "oh he did this, so he must be a Republican".
And look, no offense, but it's stuff like this that drives our parties apart, when people insist on labeling politicians DINOs or RINOs or flat out "Republican/Democrat-lite" based on one or two actions they didn't think were appropriate for a Democrat or Republican. It makes those people more likely to go further left or right to avoid this kind of stuff.
Obama supported the surge, cut entitlements, generally tries to appoint summers/geithner types if he can get away with it, supports drones/NSA spying, is against public option/si ngle payer, pro-free trade, decent but not great on environmental issues.
He's no DINO - he and Clinton are typical Democrats, and there is a substantial difference between both parties today. But it's fair to say that both parties have (gay rights aside) drifted to the right considerably with advent of the neoliberal consensus, and Obama's beliefs on the market and entitlements are broadly similar to Republicans of two decades ago, though it's not a straight equivalence.
