Maryland Gov. Martin O' Malley (D) speaks at a hearing last week in Annapolis on same-sex marriage . (Mark Gail/The Washington Post)

The gesture by Marylanders for Marriage Equality comes as the General Assembly moves toward voting on the issue, which Gov. Martin O’Malley (D) is championing during the 90-day legislative session.

In an attempt to build support for a measure that failed last year, O’Malley and his allies have been trying to court Republicans and wavering Democrats in the House of Delegates.

Marylanders for Marriage Equality said it would forward the postcards to the offices of lawmakers in coming days.

“We’ve been out in the field for months, on street corners and at Metro stops, in homes and coffee shops, encouraging folks from all walks of life to express their support for marriage equality,” said Travis Tazelaar, the group’s field director.

Monday night’s rally comes two weeks after a few hundred opponents of same-sex marriage gathered in the same location outside the State House in Annapolis.

O’Malley’s office said he is planning to drop by the rally about 6 p.m.

Other advertised speakers include Sen. Allan H. Kittleman (R-Howard), the only Republican to vote for same-sex marriage legislation last year in the Maryland Senate; and Candace Gingrich-Jones, the half-sister of GOP presidential candidate Newt Gingrich. Gingrich-Jones lives in Maryland and works for the Human Rights Campaign, a Washington-based gay-rights group.

Also scheduled to appear are two lawmakers supportive of O’Malley’s bill in the House, Del. Benjamin S. Barnes (D-Prince George’s) and Del. Keiffer J. Mitchell Jr. (D-Baltimore).