Had i had the time in "Created Equal" something like that could have happened; as it was I mention womens' suffrage occurring by 1876.
One of the problems with a POD after 1900 is that the media doesn't have any women to prominently feature and extoll the virtues of.
Margaret Chase Smith becoming mroe prominent could work, the only problem is that upon checking I realize she was born in 1897. (I was thinking about 1910 for her) Unless she somehow becomes more prominent in the '60s...and even then it's tough. You'd probably have to go back to the '50s. Perhaps she gets more credit for stemming McCarthyism, and a spot opens up in Ike's cabinet? That might inspire more media focus on her with more women then thinking, "I can do that, too."
She's still a little old for a ticket in '68, though not by much, Nixon could still pick her I suppose. And there *might* be more by the '70s who have a chance.
Another thought - could Amelia Earhardt's survival lead to her entering politics? Only problem is, she's only about 5 months older than Smith. Still, she's a very noteworthy woman who, if she were to decide her country needed her leadership in Congress after her circumnavigation, could cause people to trumpet her being a pioneer of politics just as she hd been of the air. Perhaps she's seen as having growing influence in the '40s and '50s supporting Civil Rights (being born in Kansas where Brown v. Board of Education originated might help get her some air time), and she leads a growing number of females into Congress and onto national tickets by the '70s?
Of course, the challenge isn't the earliest but just to have more, but I do think you need a pioneer who can be recognized nationally so that others see her and are inspired to follow. The more people who follow such a role moel, the more chance there is to have them on national tickets and in Congress and the Senate in general.