MEXICO CITY (Reuters) - A group of Mexican opposition senators proposed a constitutional reform on Wednesday that would enable a sitting president to face trial for corruption for up to three years after leaving office, in a bid to bolster the fight against graft.

Mechanisms exist under the constitution to prosecute public officials, from senators to state governors and supreme court judges, via impeachment and removal from office.

However, the constitution does not provide for impeachment of the president and mandates that the incumbent can only be put on trial for treason and serious crimes, such as murder.

“You can’t even touch the president with a rose petal,” said Juan Carlos Romero Hicks, a member of the center-right National Action Party and one of five senators behind the initiative.

The proposal sets out changes in the constitution that would allow a serving president to be impeached and face trial for “acts of corruption” and other crimes. The president would also be liable for any crimes committed in office for up to three years after leaving the job.

Public discontent over corruption has become a major issue in the run-up to the July 2018 presidential election.

Graft scandals within the ruling Institutional Revolutionary Party (PRI), and allegations of conflict of interest surrounding President Enrique Pena Nieto and several top aides, have battered the credibility of Mexico’s highest office.

Those in favor of preserving existing protections argue they are necessary to prevent political witch-hunts after changes of power in Mexico, where the president cannot seek re-election.

The PRI is the strongest party in Congress, making the proposal unlikely to prosper without support across the aisle.