For a president with a plunging economy and the world's fourth-highest number of confirmed COVID-19 deaths, Mexico's Andres Manuel Lopez Obrador is not doing so badly.

In his second state-of-the-union address on Tuesday, Lopez Obrador emphasised what he considers his major achievements: the fight against corruption and his government's austerity.

"This government will not be remembered for being corrupt; our principal legacy will be cleaning up Mexico's public life," he said.

The president also noted the challenges facing his administration, including the coronavirus pandemic and the struggling economy, but gave his positive take.

Mexico will emerge from the pandemic with a better public health system, and his economic strategy relying on direct support to those most in need is beginning to bear fruit, he said.

He still gets 52 percent support for a coronavirus policy that amounts to little more than damage control with as little testing as possible and almost no contact tracing or mandatory lockdowns.

It focuses instead on expanding the number of hospital beds.

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The honeymoon for Mexico's "Teflon" president is clearly over and he no longer has the near-sky-high approval ratings he once had.

According to a Reforma newspaper poll published Monday, Lopez Obrador still has a 56 percent approval rating; that is down from a peak of 78 percent in March 2019.

The poll had a margin of error of four percentage points.

Nor has Lopez Obrador's anti-crime strategy been working; murders are stuck at around 3,000 per month in the nation of almost 130 million inhabitants, about the same level as seen during the last two years.

Drug cartels continue their bloody turf battles, and cocaine flights and fentanyl pill exports continue.

"Despite the fact that people are seeing the problems we have and that the results are quite meager, they nevertheless give the president quite a high rating," said Jose Antonio Crespo, a political analyst at Mexico's Center for Economic Research and Training.

Lopez Obrador hopes his main legacy will be going after corrupt politicians who stole hundreds of millions of dollars in past administrations.

A lurid series of leaked videos and testimony in recent weeks has reinforced what most Mexicans have long thought that former administrations were full of crooks but they provide little legally admissible evidence.

Most of the accusations, including videos of politicians handling suitcases full of cash, were made by a former state oil company director who himself hopes to avoid prison.

In fact, 58 percent of those surveyed in the Reforma poll did not think the corruption cases would lead to any concrete results, while only 28 percent thought suspects would go to jail.

But with nothing to brag about on the economic front - gross domestic product dropped 18.7 percent in the second quarter - the anti-graft campaign may be a key part of Lopez Obrador's strategy for the 2021 midterm elections, in which he hopes to hold on to his bare majority in congress and win more governorships for his Morena party.