Learn English – ‘fewer than 1 in 10 Americans’

"More likely than not" logically means with a probability greater than 50%. A probability of 50% would be "as likely as not".

But the user of the phrase is not making a mathematically precise estimate of probability. They are expressing what they think is likely in an intentionally vague way, and it's misplaced precision to try to assign a number to it.

As an opposite, one could simply say "unlikely" to mean a probability of less than 50%. Or, given a proposition A, one could say "It's more likely than not that A is false."

I guess you could say "less likely than not", but I've never heard that.