Why did Ted Kennedy challenge Carter in 1980?

[J. J.]

There was also the sense that Carter was weak and could be taken out.

In December of 1979, Reagan was not the front runner.

[J. J.]

One thing not shown on the chart was Carter prior to the "hostage crisis."  Carter was much weaker, but used it to rally the country.

The "October Surprise," that didn't happen, would have been Carter getting the hostages released in late October.

Interestingly, prior to the Wisconsin  Democratic Primary, which was close, the White House announced that there would be "big news" about the hostage crisis.  After Carter defeated Kennedy, there was no "big news."