The only probable POD is for Watergate to blow up in, say, September or October.
Sure Watergate was known, and McGovern railed about it, but nobody really paid attention. Have someone make a mistake that means it leads to Nixon faster, or whatever.
Nixon's a crook on national TV, and McGovern wins by default. Likely the Watergate babies of 1974 come in '72 & '74 TTL, and McGovern has a half-decent congressional majority.
As to what happens, who knows. McGovern probably pulls out of Viet Nam, which means South Vietnam falls in '73 or '74; he may try and pass his guaranteed annual income, but it likely won't pass without Republican forced cuts in stuff like welfare and Medicare (remember that Nixon would have passed something similar if he had offered cuts in entitlements); national health insurance is a possibility as McGovern pushing for it is more palatable than Nixon pushing for it (Ted Kennedy wanted national healthcare, and so turned Nixon down); and so forth.
Overall, however, Nixon was pretty left-wing on domestic programs (because he only cared to break the Democrats, he didn't actually care about domestic issues) and so McGovern is unlikely to be much further left—at best, as I mentioned, a few programs that Nixon offered but didn't go forward get passed.
1976 rolls around and McGovern faces Reagan (nobody else in the Republican party was going to beat Reagan in '76, heck Ford only narrowly won and he was the President).
Odds are Reagan wins. McGovern only won in the ATL because Nixon imploded (presumably Nixon wins the South and a couple other states in '72), and Reagan will carry California, the South, the traditional Republican states in the Rocky Mountains and Midwest, and probably the industrial belt—which means McGovern loses.
However, Reagan has a Democratic congress (certainly the House, probably the Senate), the US is likely experiencing much of the same difficulties as they did in OTL '70s (though health insurance + NIT/GAI) is going to be a solid buffer for the poor and middle class, and so on.
Reagan, albeit with a somewhat different US, probably does most of the things we discuss in the various "Reagan wins in '76" threads scattered around—keeps Panama, if McGovern hasn't handled it already; deregulates; possibly a large tax reform that results in overall lower taxes (avoiding his OTL see-saw of tax raises and cuts over the first few years as ATL Reagan isn't a supply-sider and isn't seeking a 30% cut—so think '86 tax reform) or if not just moderate tax cuts; takes a harder line against the USSR; and so forth.