When you think about prominent 3rd party candidates in modern (post-WWII) U.S. elections, a few names come to mind: Thurmond and Wallace, who actually won electoral votes; Ross Perot, with his highly successful back-to-back campaigns in the 90s; Ralph Nader and his spoiler effect for Gore in Florida and New Hampshire. John B. Anderson, in spite of earning the 4th most votes of any third-party candidate ever, is relatively unknown. Why is that?
The 1980 election was one of a few elections to be an electoral landslide, in spite of Reagan only receiving 50.7% of the popular vote. Which candidate did Anderson draw the most votes from, and did he help Reagan achieve the electoral landslide, or did he simply reduce Reagan's popular vote share from what it would have been in a head-to-head with Carter?
Anderson was a liberal Republican Congressman (not an oxymoron at the time) from Illinois and an unsuccessful candidate for the Republican nomination for president in 1980. After Reagan beat him (among others) for the GOP nomination, Anderson launched a third party bid, trying to run a centrist campaign, to the left of Reagan but to the right of Carter. Anderson had a number of issues that distinguished him from Reagan: He supported Carter's grain embargo against the USSR, and he supported the Equal Rights Amendment (the ultimately failed effort to get gender equality written into the Constitution).
Anderson picked a two-term former governor of Wisconsin as his running mate and even qualified for the first presidential debate. Carter stayed out of the debate in protest of Anderson's inclusion, and the debate alone with Anderson ended up helping raise Reagan's profile. At the second debate, Anderson was not allowed in, and his exclusion at the second debate put Anderson's support into a tailspin.
He ended up finishing with nearly 7% of the vote. Despite being a Republican and running between the two candidates, Anderson ended up more of a spoiler for Carter, as he did best with liberal voters disappointed with Carter but nervous about Ronald Reagan: his best state was Massachusetts, where he got 15% of the vote and was clearly responsible for Reagan narrowly winning that usually liberal bastion.
I'd recommend checking out the 1980 map on uselectionsatlas.org for breakdowns of Anderson's performance down to the county level.