Beginning with the...
...1978 midterms
https://www.gocomics.com/garfield/1978/11/07This points to very low turnout in Carter's midterms. This is the POD, unless turnout was already very low at the time.
1980 General
https://www.gocomics.com/garfield/1980/11/04America decides to elect an ideological conservative who promises to clean up and reduce the size of government. Obviously Ronald Reagan.
1982 General
https://www.gocomics.com/garfield/1982/11/02One party expects a big wave in the midterms, but the other (on the defensive) benefits from the farm vote and from younger voters. The other party can only hope to improve in 1984
1984 General
https://www.gocomics.com/garfield/1984/11/06This is a strange world we live in, but you gotta play by the rules. One party tries to form a winning coalition but finds out the world has changed around them. This is clearly the Reagan vs. Mondale matchup, with Mondale and the Democrats thinking that a competitive election will occur. Mondale might not be as embarrassed as OTL, however their expectations are sky high compared to the resulting defeat.
1986 midterms
https://www.gocomics.com/garfield/1986/11/04If my predictions for OTL results for '80 and '84 hold water, then this is a Democratic wave.
1988 General
https://www.gocomics.com/garfield/1988/11/08A tossup election decided by voters, often redneck rural whites in places like southeast Ohio, making up their minds only on election day. Margins in key states are so narrow it's quite possible a one day delay or one day early could've changed the result. A populist southern Democrat could pull off a win in this scenario. It's a bit early for Trump style populism from the Republicans, so that's probably the most likely setup, although a "tough on crime" Republican could squeak by as well. Regardless this election is closer than 1976.
1989 Special Event
https://www.gocomics.com/garfield/1989/10/23The 1980s ends on a bad note for America and the world. This could be a nuclear scare or a recession worse than what occurred during the same period OTL.
1990 midterms
https://www.gocomics.com/garfield/1990/11/06This is a massive wave, probably against the incumbent President, in which the favored party gains either 12 Senate Seats or 12 Governorships. Jon's blue outfit suggests Republicans are on the losing side, remember they were blue back then, but I don't know how strong this is.
1992 General
https://www.gocomics.com/garfield/1992/11/03Whatever rage the country was going through in 1990 has dissipated back to general malaise and apathy. A result like OTL 2020: challenging party wins ("I hate you") but not by the margin they expect because nobody really shows up.
1994 midterms
https://www.gocomics.com/garfield/1994/11/08It's a dramatic election as control of congress is down to the wire. People are entertained and it makes for higher ratings for news channels. My guess is the challenging party does not gain control.
1996 General
https://www.gocomics.com/garfield/1996/11/05American democracy perks up again as tens of millions of educated voters tune in to watch a civilized debate of personal attacks and ad hominems from both sides. The dog represents the challenger and Garfield represents the incumbent President. The comic doesn't really hint on who wins though.
1998 Midterms
https://www.gocomics.com/garfield/1998/11/03Jon represents the challenging party try to scare the nation into voting them in.
2000 General
https://www.gocomics.com/garfield/2000/11/072002 midterms
https://www.gocomics.com/garfield/2002/11/052004 General
https://www.gocomics.com/garfield/2004/11/02The nation votes for a left wing Democratic administration over a Republican union buster. They may not admit it, but they'd rather be hardly working than working hard.
2006 midterms
https://www.gocomics.com/garfield/2006/11/07Several close races are decided by poor first time younger voters. They may be poor but they like the current left wing President.
2008 General
https://www.gocomics.com/garfield/2008/11/04The current President campaigns as more moderate than s/he actually is and wins a second term as a direct result.
2010 midterms
https://www.gocomics.com/garfield/2010/11/02This represents a major gain for a party in response to the party in power being perceived as condoning verbal attacks against a certain demographic. "Bigots suck!" = big evangelical turnout for the GOP.
2012 General
https://www.gocomics.com/garfield/2012/11/06The nation is angry and strongly supports the opposing party, giving them the white house, and also deposing several experienced incumbents thought to be safe.
2014 midterm
https://www.gocomics.com/garfield/2014/11/04ugh this comic sucks.
Nation tired of political theater and attack ads, I guess?!
2016 General
https://www.gocomics.com/garfield/2016/11/08Garfield is solidly declined at this point. However, this comic is clearly a young bumbling challenger getting embarrassed by the incumbent.
2018 midterms
https://www.gocomics.com/garfield/2018/11/06 (WARNING: CANCEROUS)
Mental decline and outlandish conspiracy begin to leech into American politics.
2020 General
https://www.gocomics.com/garfield/2020/11/03Democrats try and win back power with a cultural liberal. It doesn't work. Turnout is slightly lower than hoped due to bad weather.
2022 midterms
https://www.gocomics.com/garfield/2022/11/08There is a first term congressman named Leopold, and a first term congresswoman named Euphema.