It’s tough without reaching into at least the 1970s, gets easier the further back you go of course. But alright:
Paul Wellstone doesn’t have MS and butterflies from that mean he doesn’t die in a small plane crash like Begich and Litton. He runs a credible campaign that comes in second? third? but loses in 2004. In 2008 Wellstone doesn’t run for the Senate again, instead from 2005 launching himself into a multi-year project for the Presidency. It works because Obama doesn’t quite make it, in the end throwing his support behind Wellstone over Clinton after Paul takes Iowa and New Hampshire and half of Super Tuesday.
Wellstone/Obama smashes Romney/??? into the ground as Wall St and friends do their best to make capitalism a dirty word so for one of the few times in American history socialism lite looks pretty good. With a shocking defeat of Mitch McConnell in Kentucky over his wife’s Chinese business ties Republican Senators are left without their guiding star and with a solid 2 point victory by whatever Dem in Minnesota gives the Dems 62 Senators right away and 63 shortly with Obama’s replacement.
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2004:
Edwards wins re-election North Carolina as he dropped out of the Presidential with Wellstone induced butterflies on the shadow primary, Daniel Mongiardo wins in Kentucky with Paul Wellstone going into unlikely states to get coal miners / union folk going, Daschle in South Dakota does one bit better taking him into his own failed 2008 run for President and badly depleting Obama of campaign staff vs OTL.
2006:
Ford wins Tennessee with marginally stronger support of 50-state plan via Wellstone and above unlikely Dems in the Senate pushing that strategy plus helping out.
Edit: plus a few Governors I’m too lazy to check haha, but yeah just overall more Dems.
)
2008:
So. Paul Wellstone, easily the most left President since LBJ or FDR but untainted by overseas war (though ya know still stuck in a couple) & unshackled from racists. 64 Senators, but all the extra is Republican states vs OTL.
If you want a crazy ton of stuff passed the filibuster has to go. But even ITTL I sincerely doubt there’s 50 votes for it, unless the Republicans are willing to gamble hard lol— that happens and you get the even more wanked version of TTL. At least 3 Senators can be lost on any given vote though, which is massively helpful when combined with the ~4 potential Republican votes (maybe more with no Mitch, but -waggles hand-) so perhaps 7 Dems can be lost. Spector maybe-probably stays with the Republicans ITTL, if he switches take it to 64 Dem Senators and 3ish Republican votes.
Anyway filibuster intact sucks but with Wellstone and campaign infrastructure rolling the public option goes through, stimulus is spending instead of tax cuts, bankers are punished and two banks nationalized (briefly, no magic wand lol), and home owners are saved instead of screwed. Bigger reforms. Card Check. Minimum wage hikes. Lots of stuff… until 2010 when 4-8 senate seats go down and maybe the House flips too. Probably filibuster curbed on Supreme Court if needed.
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Short term, basically the theoretical max of a lefty Obama administration and a Democratic party with all the luck for a few years. ACA is much better plus public option, high speed trains here and there, maybe some postal banking, unions boosted a bit, and so on.