We're certainly too close to the next congressional elections. I think it's only fair to give people the chance to elect a new Senate before a new supreme court justice is appointed.
Moot point, regardless of what happens with the open SCotUS seat, the odds are pretty good that the Republicans retain control of the Senate in 2019. The Senate Electoral map for this cycle is extremely hostile towards the Dems(as something like half of the Democratic Senators are up for election this year).
All the Democrats achieve by throwing up roadblocks in the Senate in regards to that SCotUS seat is increasing the risk that more Republicans show up to vote in November than might have otherwise. Which in turn has the knock-on effect of potentially giving the Republicans both more Senate seats than they'd get otherwise, and thanks to a stronger Republican turnout, it'll probably help defend a few House seats as well.
So I guess this is going to be an odd political standoff. Both parties have an equal number of compelling reasons to both delay, and accelerate the nomination process for Kennedy's replacement. A lot of it comes down to the risk/reward calculus the DNC makes and how disconnected from reality they are.
My expectation is their "Reality on the ground" however is going to force most Dems to support pushing it back to next year because Anti-Trump will eat them alive otherwise.
The more decisive part will be what game the GOP decides to play with it, as they can "go nuclear" and just take care of it. If they're feeling plucky, and play it smart, they'll let the Democrats delay it into next year and make it a "Get out the Vote" issue for Republicans. Anti-Trump was going to vote regardless, so SCotUS doesn't matter for them.