NPR and PBS stations nationwide are rallying their audiences to contact Congress to fight against Republicans’ proposed spending cuts, but some affiliates’ pleas may violate laws preventing nonprofits or government-funded groups from lobbying.

Interrupting popular programs, the stations air warnings that cuts could end beloved children’s television shows such as “Sesame Street.” Some stations urge their audience to call and let Congress know their feelings, while others go further, instructing viewers to “stop the Senate” or “defend federal funding” for public broadcasting.

The ad campaigns are a direct response to House Republicans’ push to eliminate all Corporation for Public Broadcasting funds for the rest of the fiscal year. Democrats have fought the cuts and President Obama asked for $451 million for CPB in his 2012 budget request — a $6 million increase...