DENVER — Marijuana proponents scored significant victories on Tuesday as voters around the country passed ballot measures decriminalizing marijuana possession and approved regulatory taxes on the drug.

In Colorado, voters backed a heavy tax on recreational marijuana, which was made legal here last year. The tax will pay for the cost of overseeing the state’s marijuana industry as well as school construction.

Voters in three Michigan cities approved measures legalizing the possession of up to an ounce of marijuana by adults on private property, following Detroit and Flint, which passed similar measures last year. And voters in Portland, Me., passed an ordinance legalizing the possession of up to 2.5 ounces of marijuana by adults over 21, making it the first East Coast city to pass such a law, advocates said.

The victories are widely seen as fuel for the legalization movement, which has chipped away at state drug laws over the past decade and has vowed to push for more changes from state legislatures.