"I don't know if this is true at all. I don't think Truman, Ford, or Bush 41 ever served in the U.S. Senate."
Yes, they were all senators.
"And even if they all did, you'd also have to consider that McGovern, Mondale, Dole, Gore, and Kerry"
So what? Some won, some lost. That's not a curse, that's a normal distribution.
Gerald Rudolph Ford, Jr., was never a senator. George Herbert Walker Bush was never a senator but LOST two Senate races: 1964 and 1970, to Yarborough and Bentsen, respectively. HST was of course a U.S. senator but not well-known until the convention tapped him in 1944.
I'll do some more research, but I can tell you right now that George Bush #41 never served in the U.S. Senate. He ran unsuccessfully for the Senate in 1964 and in 1970, and later served as UN Ambassador and CIA director before he was picked as Reagan's running mate in 1980. His father served as a U.S. Senator back in the 1940s or 1950s.