If these donors are not buying favors, they would have no reason to reject the clearinghouse concept. Some people might find this idea too na ve to work. The donors simply would tell the candidates of their generous support. But the key to the newly proposed system is that donors would not be able to prove to whom they made the contributions. Candidates would not know if these donors claims of campaign contributions were true.
Political candidates are anything but na ve.
They all know that some donors want to buy political favors with campaign contributions. Candidates surely would realize that anyone who was sufficiently unscrupulous to try to buy influence would have no qualms about lying about whether they made a donation or to whom it was given. The candidate could not know whether the donor really had contributed to his or her campaign. Even if a donor had a canceled check made out to the clearinghouse, or a receipt from the clearinghouse, candidates would know that this donor could have contributed to any candidate or to any political party. Someone unprincipled enough to try to buy political favors might donate to your opponent and then lie to you, saying the donation was to your campaign.