Iowa Caucus is scheduled Monday, Feb 3rd.
New Hampshire is Tuesday, Feb 11.
Senators running still in the primary:
Elizabeth Warren
Bernie Sanders
Amy Klobuchar
And, if one of the drop-outs, like Corey Booker or Kamala Harris, are tapped by the nominee as a running mate, they can also be tied up in a prolonged impeachment trial -- or even with multiple trials if the House keeps up their shenanigans -- while trying to campaign.
When the Senate is involved in an impeachment trial, the Senate is not allowed to take up any new issues until impeachment proceedings are completed. They are also required to be in attendance, meaning they can't be off campaigning, holding rallies and fundraisers, or doing talk shows and returning to DC just to cast a vote once in awhile.
As it stands, one could argue that Pelosi through impeachment, while possibly trying to help Trump lose in November, could also be preventing the Dem frontrunners , 3 of them Senators, from winning, too.