I'm not completely sure about that.
Best case scenario: Blocking Obama's nominee coupled with a conservative win in the Presidential election and holding the Senate will allow the conservatives to put in someone like Scalia.
Worst case scenario: Blocking Obama's nominee only to have Hillary win and the Democrats recapture the Senate will allow Hillary to appoint the most liberal nominee ever.
Allowing Obama to nominate someone but making it clear that only someone who is no worse than a TRUE moderate will have a chance to be confirmed isn't as bad as the worst case scenario.
“Worst case scenario: Blocking Obama’s nominee only to have Hillary win and the Democrats recapture the Senate will allow Hillary to appoint the most liberal nominee ever.”
I don’t believe this is a practical concern. Voting on Supreme Court nominees requires first a cloture vote (60/40). There is no way dems come out of the Nov election with 60 seats.
Scalia was a known true conservative and an originalist. There is substantial precedent in supreme court nominees that you do not deviate too far from the politics of the justice being replaced. All parties recognize that something close to a balance is necessary. So probably the worst case scenario would be a left leaning moderate.