Activist Headquarters

...is probably a good idea. As for how much they pass on to campaigns, I did find this on the ActBlue web site:

"We pass along a 3.95% processing fee on contributions to the groups using our platform.

Completing a contribution involves expenses to process your credit card. We're legally required to pass along processing costs to the campaign so that we do not make in-kind contributions to them.

As a non-profit, we rely on tips from donors to pay our bills."

That sounds to me like they pass on most of the donation to the campaign.

If I donate through an organization I make sure it's well established with a proven track record.

I admit I'm not crazy about my info being shared. I get tired of deleting 20-30 emails asking for money every day during campaign season but if it helps us win elections, it's a small price to pay.