cos

ActBlue is a portal for donations to candidates, elected officials, and organizations. That ActBlue page is not for donating to ActBlue, it's for donating directly to the 65 House Progressives who have signed the letter. I think that would be most effective - if you then call the ones you gave to, and tell them you did so and why. (although the total on that actblue page is getting impressive and sends a message on its own, too)

If you want to additionally donate money to an organization doing good organizing and lobbying work on this, I'd say Democracy for America. Firedoglake and MoveOn are good too.