The first time Tammy Baldwin decided to run for Congress in the late 1990s, her fellow Democrats, who’d publicly championed gay rights, worried behind the scenes that Wisconsin voters might prefer a straight candidate to one who was openly lesbian. “I remember thinking how hypocritical of us, and completely contrary to the equality we said we stood for,’’ says Massachusetts Congressman Braney Frank. “I wouldn’t have it. I said - and I wasn’t alone in making this argument - we have to back her. We can’t allow who she is to be held against her. And we can’t hold against her. That was 1998. Now Baldwin is running for Senate in Wisconsin with no primary challengers in her own party. A lot has changed.’’ Frank’s reflection about Baldwin’s support from her peers and her political will comes days after he announced on Nov. 28 that he plans to step down from Congress after 31 years, and suggests that perhaps his most important political legacy may be that he helped to normalize being openly gay in public office. “Next Memorial Day will mark 25 years to the day that I publicly came out, came out in a Boston Globe story, in fact,’’ Frank said recently. “And while I did it primarily for myself, I also wanted to make a point that this was me, this was a part of me, and this was not going to be used to hurt me.’’