John McCain ought to become a Democrat - and a presumptive front-runner for their party's presidential nomination in 2004.
I'm second to no one in my belief that totebagging Democrats are overly swayed by all the bipartisan bullshit, but, no, there was no way John McCain would have been "a presumptive front-runner" in the Dem primary. I don't know why people like to pretend that every presidential primary contest isn't a nasty battle* between competing factions who, given that they are all crazy people who think they should run the world, aren't going to (and never do) play nice unless they think playing nice will get a couple more Iowa voters, and even hapless Democrats could have nuked John McCain with one television ad because John McCain had a very long history of being a Republican. A very conservative Republican who often showboated for gullible marks like Jon Chait and other wannabe tire swingers, and very occasionally did an actual mavericky thing like almost all politicians, but still a very conservative Republican.
Generally there's always a push from centrist types for Democrats to put a Republican on the ticket, and a lot of concern trolling from Republicans telling us about the one Democrat that maybe they could vote for (but won't). No one ever tells Republicans to run a "unity ticket" and Professional Democrats don't pitch editors for pieces about Who Republicans Have To Nominate If They Want To Win because that's stupid. So is the reverse, which happens all the time.
*the least nasty primary in my lifetime was in 2016, even if people being assholes on the internet made it seem otherwise.