Andrés Manuel López Obrador
This explosive news was shared by Reuters, which in turn was reporting on a mid-December video of a speech that McMaster gave to the Jamestown Foundation and which was just posted on a Mexican journalist's Twitter account over the weekend. In it, one of the US' most influential security figures says in relation to the unsubstantiated claims of Russian interference in foreign elections that "you've seen, actually, initial signs of it in the Mexican presidential campaign already", but to Reuters' credit they added that he didn't elaborate on this afterwards and even cited an expert who remarked that "so far, it's just speculation".
That said, Reuters attempted to propel the paranoia forward by remarking that the leftist populist-nationalist candidate Andres Manuel Lopez Obrador, commonly known by his abbreviated initials as AMLO, is "seen by some analysts as the Kremlin's favorite, given the positive coverage he has received from government-funded media outlets like Sputnik and Russia Today", thus relying on the conspiracy theory that everything on Russia's publicly funded international media outlets is apparently aired on direct orders of the Kremlin, which isn't true at all. Nor, for that matter, is the inference in the report that Russia is backing AMLO because of its desire to sow problems between the US and Mexico.
There's credence to the forecast that AMLO's potential victory would complicate US-Mexican relations because of the contradictory visions of their two leaders in that case, but this, as well as Russian international media's detailed coverage of the leftist populist-nationalist candidate, don't in and of themselves "prove" anything about Moscow's alleged meddling in the upcoming elections. Rather, McMaster's hysterical claim seems to be part of a preemptive infowar designed to discredit AMLO's potential victory just like the Clintons tried to do with Trump's over the same issue of alleged "Russian interference", thereby suggesting that the US' permanent military, intelligence, and diplomatic bureaucracies (or "deep state") assess his odds of winning to be much higher than is publicly being reported.
That would explain why one of the top decision makers in the Trump Administration is already rolling out the weaponized narrative that Russia is supposedly backing AMLO, since they also want to add ammunition to the right-wing's arsenal of personalized attacks in order to scare the electorate away from voting for him. Thus, it's actually the US that's openly interfering in the Mexican elections, just as it always has to one extent or another, and not Russia, with McMaster's clumsily blatant hypocrisy emphasizing just how high Washington believes the geostrategic stakes to its security to be if "the wrong guy" gets into power and how desperate the US is to stop that from happening.
Comment: If a candidate is pro-Russian in any way, he or she must be a Russian agent. There is no other explanation. If voters agree with the Russian point of view on any topic, they must be victims of Russian propaganda. There is no other explanation. If a "pro-Russian" candidate wins an election, it must have been as a result of Russian meddling. There is no other explanation. These are the key tenets of Russian meddling.
Western opinion makers have set up the perfect system. They lie, make sure only lies get any air time, then slam anyone who points out the lie or tells the truth as a Russian tool, simply because Russian sources also point out the lie or tell the truth. Since the truth is not available from Western sources, the source of such insidious truth must come from Russia. And that's bad. Even though it's the truth. Doesn't make sense? Maybe you're a victim of Russian mind control.
See also: Euroskeptic Zeman leads in Czech polls amid anti-Russian hysteria about "Russian meddling"