This is what they call "dirty tricks", or more colorful labels:

[LongtimeAZDem]

Link to tweet

Yes, King was stupid to discuss it, but the real issue is that "rival campaigns" put it out there to throw shit at Biden.

King should reveal who gave him the info; lying should not be protected by the reporting shield.

[Mike 03]

1. Agree with you.

Voters deserve to know which campaign made this false allegation.

[tishaLA]

2. John King screwed up the Boston Marathon bomber story

He's not the most trustworthy reporter IMO

[OneMoreCupOfCoffee]

3. Oh I wonder who his "source" might be?

[LongtimeAZDem]

4. No way to know, unless he fesses up. Might be ours, might be theirs.

[OneMoreCupOfCoffee]

6. Occam's razor

[LongtimeAZDem]

8. I tend to think so, but I won't accuse anyone without proof.

[OneMoreCupOfCoffee]

9. And I will never give notorious dirty-ticksters the benefit of the doubt.

[DanTex]

5. Probably the same "rival campaigns" that tried to smear Bernie before the caucus.

https://www.democraticunderground.com/1287464974

[Kingofalldems]

7. Gatekeepers loved that fake story.

[redstatebluegirl]

10. I think we know who put it out there.