PORTLAND — Mainers United for Marriage, the political-action committee formed to support a referendum asking Mainers if they favor legalizing gay marriage, is launching a door-to-door campaign this week.

The committee is also expected to announce a donation to the campaign on Monday. Three of four campaign offices — in Portland, Brewer and Lewiston — will open Tuesday.

The committee will also organize a statewide volunteer day across the state Saturday, according to organizers.