Imagine one of your co-workers tries to get you fired on Monday. He fails. You keep your job. On Tuesday, you're forced to sit down with that same co-worker to figure out your department's budget for the next year. You'd probably come into that meeting bitter, angry and not exactly primed for problem solving.
This is the dynamic that exists when incumbent members of Congress actively campaign against incumbents from another party. When one member starts aggressively working for another's defeat, it destroys the trust that is so necessary to get anything done.
In years past, there were informal customs that discouraged this. For example, it was frowned upon for one party leader to campaign against a leader of the opposite party. But those customs have been ignored over the last decade, which has set off a cycle of mistrust and retribution that has been difficult to stop.
When members of Congress can't work together because of personal animosity, it's the American people who suffer. That's why incumbents from one party should not conduct negative campaigns against sitting members of the opposing party. That means no appearing in negative ads, no signing nasty direct mail letters and no traveling to an incumbent's district or state to play attack dog. Members would of course be free to campaign or fundraise in support of candidates from their party.
This proposal simply requires members of Congress to individually change their behavior.