Oh, those wacky socialists! They are so picky about which right-wing judge Trump will get onto the Supreme Court!
Actually, I suppose the libs are playing a long game. It’s not just about rejecting Kavanaugh for yet another conservative Trump nominee. If they can keep Trump from getting any nomination approved before the mid-terms, there is an outside chance (about 1 in 3, per 538) that they can take over the majority status in the Senate. If that happens, they may be able to keep a Trump nominee out of SCOTUS forever, by playing the Mitch McConnell game and waiting for Trump’s term to expire. And then they need to take the White House in 2020, although their current roster of candidates is not looking that strong.
And they have to hope that Mitch McConnell won’t play hardball if Kavanaugh fails, by rounding up his 51 votes before the next Congress is seated, then rushing the next nominee through the process with only perfunctory consideration. And McConnell, that wily fox, has historically been more than willing to play hardball!
The vague hope for that whole concatenation of circumstances (delay, avoid getting railroaded, take the Senate, take the White House) is really a long shot, but without it … hey, they got nothin’.