Let's begin with the decision. Donnelly, as a moderate, is often looked to in the Senate as a swing vote. Sometimes he sides with Republicans. Many times he sides with compromise. That draws the ire of the progressive wing of Indiana Democrats who wish that he would fall more in line with liberal leanings.
That's just not Joe Donnelly. It never has been, and it never will. Joe Donnelly is going to make you mad if you're a progressive, and he's going to surprise you, too. In this case, he's casting his vote with lots in mind.
First of all, there's 2018 to worry about. It will be a midterm election, and Donnelly is seen as one of the most vulnerable Senators. The vote to confirm Gorsuch takes an entire attack angle off the table for the Indiana Republican Party. The GOP can't beat him over the head with this vote like the NRA is trying to do with his gun control votes even though he voted for not only the Democratic gun control measures, but the Republican-supported ones too.
Secondly, there's a good chance that even with Donnelly's vote that Mitch McConnell won't have the 60 votes necessary for cloture and thus the Democrats will successfully have filibustered Gorsuch's confirmation. The only way to confirm him is for McConnell to overturn the filibuster and use the so-called "nuclear option" and take a simple majority for Gorsuch to be confirmed. Either way, it looks bad for the Republicans and McConnell.
Finally, Donnelly is standing up for the old rules of the Senate. Before we reached this hyperpartisan era in politics, things used to be more collegial. The President would nominate someone to a position that he believes is qualified. It's up to the Senate to advise and consent to that nomination. If someone is not qualified, the Senate is fully justified to vote down the nomination. If, however, like Gorsuch, someone is qualified for the job, then the Senate votes to consent to the nomination even if there are philosophical differences. This is how a judge like Ruth Bader Ginsburg was confirmed, 96-3.
I know that many liberals get upset when I point out the alternative to Donnelly in 2018 likely will be a Republican who is diametrically opposed to many of the things liberals support. That always must be remembered. Donnelly's balancing precariously right now. He's going to need every vote he can get.
That's the choice liberals will have to make: voting on principle vs. quite probably losing the Senate seat. I'm just being real.