Wow I was so confident he wouldn't go for another white guy. Kinda sucks.
Buddy, it's bait. the Senate is in a no win scenario. They have two options:
1. Deny having a serious nomination process for a 63 year old moderate who will serve a comparatively short tenure and isn't some boogie man. This makes them look like jackasses to the majority of America, damaging their chances across the board at keeping the senate after November.
2. Have a hearing, appoint Garland, see the SCOTUS take a decidedly left leaning 5-4 slant with Kennedy and Roberts more concerned with jurisprudence than political stance. The base hates them even more than they already do and either primary them if their primary hasn't come up yet, or protest vote/abstain against them in the election.
Either way they're going to lose politically, #2 would also rile up a serious chunk of their special interest group base since Garland has a legitimate gun control record that will make the NRA and co. freak out.
Sandoval would have been a risky pick as he was a moderate conservative and the payoffs were all the same as Garland. Garland is a moderate progressive with a few red flags that make him anathema to large sets of the GOP base however, so he is only marginally more appoint-able for the senate than a far left choice but with the ability to deal far more damage with moderates when they don't.
The real coup de gras here is that Obama chose gun control as one of the biggest GOP red flags. Mainstream America WANTS more gun control. If the senate even tries to hold a disingenuous review of Garland they'll end up attacking him for supporting gun control (they can't help it, the Ted Cruz's of their party won't be able to keep their mouths shut) and turn it into even more political fodder than simply denying the guy out of hand.
It's second term Obama to a 't'. A pragmatic choice that puts all the political pressure on the opposition now faced with a lose/lose more proposition. This is what the GOP gets after going HAM on obstruction following 2010.