They not only try, they succeed. Democrats desire to operate as much as possible unopposed. I believe they began this tactic in earnest during the Clinton years. Remember when Gingrich was under continuous investigation by the (oxymoronic) House Ethics Committee? He had to agree to keep his mouth shut in public while the Dems blasted him every chance they got with allegations never proven. Democrats are bullies, afraid of a fair fight.
When Hillary ran for the Senate, she ducked the electorate, attending only carefully scripted events where her handlers knew the reception would be positive. Remember the baseball playoffs and World Series? Mets vs Yankees, and her the big Yankee fan can't find time in her schedule to attend even one inning of one game. And the vaunted New York press never uttered a peep.
Mel Carnahan dies in a plane crash and John Ashcroft is forced to abandon his campaign, meanwhile the Missouri press and Democrats are free to continue theirs. Want irony? This week, Jim Talent's dad dies. He halts his campaign to grieve. The widow continues to pummel Jim with both fists in campaign appearances.
Need more? NJ Senate race: Lautenberg won't come out and debate. And now Wellstone's death, and Dems want Republicans to shut up while they continue campaigning. I repeat, Democrats are bullies that want to operate unopposed.