Following the split 50-50 Senate in 2000, Harry Reid flipped Senator Jim Jeffords of Vermont from being a Republican to a Democrat-Caucusing-Independent. Reid also reached out to Lincoln Chafee and John McCain however, both of whom considered the offer but didn't go all the way once Jeffords flipped (McCain in particular did a lot of defending of Jeffords).
After a special election the GOP retook the bare majority (50-50 plus Cheney as tiebreaker).
What if all three had flipped and the Senate ended up 53-47?
Could McCain have run D in 2004?