They’re trapped. If they vote “yes” they anger their constituents and get tossed out of office. If they vote “no” they anger the moonbat base and get cut off from Democrat re-election funds and they still get tossed out of office.
This is one vote they would rather duck. The Democrat leadership can allow them to vote “present” or just skip the vote.
If it failed, the moonbats would be up in arms but they can always impeach Trump again down the road.
Letting Trump position himself as a martyr is a fools’ errand and there’s no point in doing it when the Senate will acquit him anyway.
Why bother? So much political pain for zero gain.