The Missouri Democrat said her party has probably oversold the legislation that just became law.
"The side on which I'm on, that voted for the bill, probably is overpromising, [has] not been clear enough about the fact that this is going to be an incremental approach over time, [and] the benefits aren't going to be felt by most Americans immediately," McCaskill told MSNBC's Mornine Joe.
Nevertheless, the Republicans have been over the top in demonizing the health bill, McCaskill said.
"Chicken little is running the Republican party right now, and when the sky doesn't fall then all those independent voters out there go, 'Gosh, they sure told us it was going to be terrible and this isn't so bad," she said.
"So the truth, as always, is somewhere in the middle."