they can continue to be employed by any news organization.
Once they announce, however, they must resign from the network and only appear as "news items". Anything else invokes the Political Advertising rules (
http://www.fcc.gov/mb/policy/political/candrule.htm), aka "Equal time provisions" which, in a nut shell, says that if a broadcaster makes X amount of time available to Candidate A, they
must make the same time, in amount and quality (so no giving "A" prime time and "B" graveyard time) to any other qualified candidate for the same office at the same "price" (and if "A" is being paid as a commentator, the case could be made that "B" should also be paid).
This is all covered by the above link and cable outlets are specifically covered under §76.
This situation is nothing new. it happens quite often in local races where a local media personality runs for office. It can get
really complicated when "Bob Miller", owner of a local car dealership, runs for city council and his advertising campaign for his auto dealership has him appearing all the ads. Is that an "equal time" situation? even if he is not actively campaigning in the commercial just hawking his dealership?
I know that at a station I worked for the decision was made that "Bob" could no longer actually appear in his commercials while running for office..any and all commercials featuring "Bob" were pulled and his money carried over to post-election advertising. (he lost BTW and lost BIG)