10 Democratic Senators are running for reelection in states that Trump won. (Why?)
In 2016, Antonin Scalia died unexpectedly, Pres. Obama nominated a qualified moderate to try to entice the Republican Senate to confirm, but Sen. McConnell and the normally spineless, risk averse Republicans instead chose to push the Supreme Court question into the Presidential election. Why? Because these questions favor our side. It's hard to tell by their other votes sometimes and positions on issues but people still want the constitution upheld.
Now we face a very similar situation. Timing-wise, this election and this confirmation have now just merged.
If you are Joe Manchin, Heidi Heitkamp, Joe Donnelly, Bill Nelson, Claire McCaskill, Sherrod Brown, Jon Tester, Tammy Baldwin, Bob Casey, Debbie Stabenow, you have a choice to make. Support the principles of your party or the principles of your constituents in your state.
The two Republicans considered iffy on confirmation, Susan Collins and Lisa Murkowski are not up for reelection. Additionally, Rand Paul may oppose Kavanaugh on NSA privacy issues. If any or all of these R's defect, that puts even more pressure on the 'red state' Democrats up for reelection.
For the vulnerable Dems, if they oppose the President they blow the whole story they bring to the electorate about their independence from their unpopular party and Trump and their Republican opponents will be energized. If they succeed in defeating this nominee, there will not be time to run a new nomination through before the election and they could lose their seat and Trump and the new Senate could put the same great nominee through again - with ease. In other words, nothing accomplished for Heitkamp et al. There will probably never be a Dem Senator again from North Dakota is she screws this up, and consequences and effects in the other states as well.
To this discussion I would add so called blue states like MN where Trump lost by only one point and his popularity has risen since then, we have both Dem Senators up for reelection. Trump won 5 out of 8 congressional districts in MN and 78 out of 87 counties. These two Senators will vote no reflexively and will win anyway, but their support for only representing only roughly half of their constituents will be fully exposed.