Once you donate, the begging doesn't stop until you request to be removed from the list. You'll still get random requests.
Early donations are extremely valuable to campaigns. Successful campaign fundraising suggests momentum and community support. "Look, people like us! You should too." Early donations allow the campaigns to make earlier ad buys at much cheaper rates. Of course, early donations are also used for fundraising.
P.S. Early donors are more likely to be "heard" by the candidate when the donor raises concerns about the campaign, a platform, or whatnot. Donate $500 or more and chances are pretty good that the CANDIDATE will make the next fundraising call to you.
P.P.S. In the worst of political times (like now), it feels pretty good to make a donation of $100 or $50 or even $20 to a couple of campaigns that give you hope. I'm not a marcher, but I promptly donate to democratic candidates who are about fighting, not just speechifying outrage.