Do you really think dems believe impeachment will get Trump out of office?
Are you saying the Republican senate won't convict, or are you saying that Trump will try to lock himself in the Oval Office?
If it's the former, you might ant to notice that impeachment is popular in the US right now. And if they don't convict, that hands another 3 senate seats in 2020 to the Democrats (Republican senators in blue states).
And even if impeachment isn't popular, it's still the right thing to do. To moral absolutists like liberals, doing the right thing matters. Don't wait for popular opinion. Shape popular opinion by leading.