A 1916 Saxon roadster painted suffrage yellow rolled up to the Saturday Afternoon Club in Santa Rosa on Thursday afternoon, carrying two of the best-known names in the modern women's history movement and an argument about an amendment a century in the making.

The car is the centerpiece of Driving the Vote for Equality, a national tour pressing the case that the Equal Rights Amendment — ratified by 38 states and stuck for years on a procedural question about a deadline — has only one path left: back through Congress.

That path narrowed two weeks ago. On April 21, a federal judge in Boston dismissed the last open lawsuit asking the courts to recognize the ERA as ratified. Virginia, Nevada and Illinois — the three states that ratified after 2017 — had each gone to court asking federal judges to order the Archivist of the United States to certify and publish the amendment as the 28th Amendment. The Boston ruling ended the last of those suits.

The way back to Congress means either a new resolution dropping the original 1972 deadline, or a full restart — two-thirds of both chambers and 38 states ratifying all over again.

The amendment itself is 24 words, written in 1923 and approved by two-thirds of Congress in 1972: "Equality of rights under the law shall not be denied or abridged by the United States or by any State on account of sex."

In practice, it would bar federal and state governments from treating people differently under the law because of their gender. It would be the first time the Constitution explicitly guaranteed equal rights based on sex. Whether it counts as the 28th Amendment turns on a deadline written into the introductory language of the 1972 resolution, not the amendment itself.

On board the Saxon were Molly Murphy MacGregor and Kathy Bonk.

Murphy MacGregor, working out of Sonoma State University and the Sonoma County Commission on the Status of Women in 1977, started the local effort that became National Women's History Month. President Jimmy Carter signed the first presidential proclamation in 1980, and Congress codified it in March 1987.

Bonk, the Washington, D.C.-based organizer who put the tour together, has spent four decades as a strategist for women's rights causes in the nation's capital.

"We've only been at this for 46 years," MacGregor told the room in Santa Rosa. She is 74. Her daughter is 44. Her granddaughter is 14. "I'm always telling them: you need to understand what you have now that is at risk if we don't get the Equal Rights Amendment passed."