Participating in a debate in the Rajya Sabha, party leader Sitaram Yechury said that the bill was seeking to tackle the "demand side" of corruption but not the "supply side".
Yechury said Prime Minister Manmohan Singh referred to "supply and demand side" in his speeches concerning economy and inflation.
"The source of corruption should also be tackled," he said, and demanded electoral reforms vis-a-vis corporate donations to political parties.
He said the Lokpal bill should cover cases where a corporate entity seeks to influence government policy through corrupt means.
"If a (government) contract is acquired through corrupt means, how can you have it out of Lokpal?" Yechury asked.
The Communist Party of India-Marxist leader said this was "a serious lacuna" in the bill.
Yechury said public private partnership (PPP) projects should be under the ambit of Lokpal.
The CPI-M leader also said charitable institutions should not be kept out of the purview of Lokpal.