The ANC in the Western Cape said Public Protector Thuli Madonsela had been intimidated by Western Cape Premier Helen Zille, according to a Sunday newspaper.
On Friday, Madonsela cleared Zille's government of acting unlawfully in awarding a communications tender.
ANC provincial secretary Songezo Mjongile told the Sunday Times that the party was disappointed by Madonsela's findings.
Substantial issues had been watered down, he said.
"Clearly the intimidation of the public protector by the premier has influenced this particular outcome," he said.
The party was considering legal action, the weekly reported.
Madonsela said on Friday there was nothing unlawful about the Western Cape government's awarding of a R70-million contract to advertising agency TBWA.
She made four findings of maladministration, but did not recommend that the contract be cancelled.
"I haven't made a finding of unlawfulness," she told reporters in Pretoria.
Madonsela found that the tender referred to in the report was advertised, contrary to allegations that it had not been.
With regard to maladministration, she said it was ill-advised to have two of Zille's political advisers on the bid committee for the branding contract. This raised suspicions of political interference.
The protector said the premier's department had failed to employ proper demand management processes and had wasted over R8696 of public money when two out of three advertisements had to be cancelled because of this.
Zille took issue with this finding, and said the money spent on re-advertising the branding contract should be seen in context.
"We disagree [with Madonsela] that this constituted fruitless and wasteful expenditure, because it must be seen relative to the cost of not re-advertising and potentially compromising the process further," she said.
"To put this amount into perspective, it is about the same we spent flying officials to the Public Protector's media briefing in Pretoria this morning."
Madonsela received four complaints about the tender, from Mjongile, Sulyman Stellenboom, the Congress of SA Trade Unions' Western Cape provincial secretary Tony Ehrenreich, and the civil society organisation Ndifuna Ukwazi.
Zille said the report found no corruption and political involvement, interference or manipulation in the procurement process.
It also found that she was not personally involved in the procurement process, but in the decision to cap expenditure on communication services in the province at R70-million a year.
She said it was found that the presence of the two special advisers — Ryan Coetzee and Gavin Davis — on the tender bid evaluation committee had made no difference to the outcome.
Madonsela found that some administrative processes in managing transversal tenders, those which involved various government departments and strict Treasury regulations, were faulty and that they lacked capacity to manage them properly.
Zille said the provincial treasury had already picked up these faults and that steps were taken to rectify them.
She said the time and money spent on investigating the tender amounted to hundreds of thousands of rands, which could have been better spent on service delivery.
"The entire exercise was a storm in a teacup, stirred up by our political opponents," said Zille.