NEW HAVEN, Conn. (AP) — New York Mayor Michael Bloomberg is stepping up his gun control campaign by asking donors not to support four Democratic senators who opposed a bill to expand background checks.

Bloomberg's made the request in a letter to about 1,100 New York City residents. It comes as a mayor's group that he co-founded plans a national bus tour aimed at getting senators who voted against the proposed legislation in April to change their minds.

The tour leaves Friday from Newtown, Conn., where a gunman six months ago killed 20 children and six adults at an elementary school.

Erica Lafferty, whose mother was the principal of the school and was among those killed, vowed to keep up the pressure as well. She spoke to reporters Wednesday on a conference call.