Mike Gabbard Handout photo

"I can be much more effective as a legislator being a member of the majority party, and that's what it's all about," Gabbard said at the signing on the second floor of the Ward Warehouse.

Gabbard, who was elected to the Senate as a Republican last year, met recently with Democratic activists as he mulled the switch. Democrats have a 20-to-5 advantage over Republicans in the Senate, and two Republican senators are facing significant re-election challenges next year.

Gabbard — now D-19th (Kapolei, Makakilo, Waikele) — a former Honolulu city councilman, was more likely than other Senate Republicans to cross over and vote with Democrats last session. He also disappointed Republicans when he was the deciding vote to override Gov. Linda Lingle's veto of a bill that created an advisory council to help select appointments to the University of Hawai'i Board of Regents.

During the 1990s, Gabbard was as a leader in the campaign to ban gay marriages in Hawai'i.

Mike McCartney, the party's chairman, said within the party reaction to Gabbard's switch has been mixed.

Some progressive activists have raised objections, given Gabbard's history and the fact that the party's platform calls for civil unions.