IIRC, Gore had become pro-choice prior to his 1988 run. He was perceived as the less liberal candidate in 1988, but he was clearly a national Democrat; much more so than Jimmy Carter prior to 1973. Boren was more conservative than Gore, and, possibly, a wee bit more conservative than Lloyd Bentsen, the actual 1988 VP nominee, but he was a guy who at least endorsed the national ticket; he knew that disowning the national ticket would be the death knell of any national ambitions he may have.
I doubt there would have been a third party or liberal independent movement of any kind. Gore would have been acceptable to liberal activists in ways that Carter never was. More importantly, the Democrats were desperate in 1988 in ways they hadn't been since the 1920s. The Democrats were where the GOP is now; they dominated state and local politics, but could not elect a President due to the lack of fluidity in the Electoral College. Indeed, they were worse off than the GOP is now because of how far behind they were the GOP in terms of a base vote. After 1984, it could be said with accuracy that the Presidential base of the Democratic Party didn't go beyond the District of Columbia. Had Mondale been from a more Republican state than Minnesota, the GOP may have been looking at a 50 state sweep.
In that environment, no serious Democrat would have ever attempted a third party run. No one would have wanted to be responsible for the debacle that would have ensued in a year when the Democrats had a chance to win. Which Democrat would have attempted this silly idea? McGovern? I doubt it? McCarthy? He was irrelevant at the time.
I do think that the Democrats, with Boren on the ticket, would have carried Oklahoma in 1988. It's hard to believe now, but Oklahoma was around the national average for Dukakis; maybe a little bit below, but it was more Democratic at the local level, and Dukakis actually carried a Congressional district in OK and barely lost another. (The slump in agriculture during the Reagan years was an issue.) I don't think that this ticket would have carried all of the New England states, as Bush was still considered a moderate Republican in 1988 and states like MI, NJ, and OH had suburban voters that were opposed to busing to achieve racial balance in schools. Politics was different then, and less ideological.
^^^This. That's why I don't see conservative voters defecting from the GOP in any significant numbers in 2016 if the nominee, say Jeb Bush, nominates Portman or Collins for VP. It's also why I thought Palin was such a drag on McCain in '08.
