From one angle, Senate Republicans just made a massive mistake. They could have simply waited for President Obama to nominate a replacement for Justice Scalia, drawn out the confirmation process, and then rejected the nominee on ideological grounds months from now.
But Senate Republicans didn't do that. Instead, Senate Majority Leader Mitch McConnell immediately announced he wouldn't consider any nominee from Obama no matter how qualified, how conservative, or how beloved. What could've been a debate over the noxious liberalism of Obama's choice has now become a debate over the reflexive obstructionism of Senate Republicans.
The smarter GOP play: Be skeptical now and wait to oppose a specific nominee. Pavlovian opposition makes the D argument so much easier— Dan Pfeiffer (@danpfeiffer) February 14, 2016
What's worse is that McConnell's decision leaves the GOP with few options if their situation deteriorates through the fall. It's entirely possible that six months from now, Donald Trump will be the Republican nominee and Republicans will be in serious danger of losing the Senate. If that happens, Republicans will wish they had cleared the compromise candidate they could have forced on Obama now, rather than watching President Clinton and Majority Leader Chuck Schumer nominate a liberal they can't stand to tip the balance of the Court. McConnell could have left himself that option, but he didn't.
From another angle, though, McConnell and the Senate Republicans are doing the obviously rational thing. Most Senate Republicans represent red states, and most of them aren't even on the ballot this cycle. Their proximate fear isn't the general election in 2016, or even the composition of the Supreme Court. As political scientist Dave Hopkins explained on Twitter, it's their next primary election, and the voters in that election would see compromise with Obama as an unforgivable sin, even if it was a wiser strategy:
From this perspective, McConnell did the only thing he could do. The optics might be bad, and for conservatives, the eventual outcome might be disastrous, but this was a primary election litmus test, and McConnell needed to make sure his caucus passed it.