"The religious right is in steep decline. "

That's when they become most dangerous.  Cromwell's Puritans were not in the majority when they imposed a Taliban-like regime on England and Ireland.  The Iranian mullahs were not in the majority when they seized power in Tehran.  The teetotallers who gave the US Prohibition were not a majority of the population.

A tightly organized group of true believers, willing to use violence and intimidation to enforce their world view, can overwhelm a populace that is not organized and determined to defeat them.  That's how fascist and communist movements have taken power in many countries.

There is no suggestion in The Handmaid's Tale that the religious right took power in Gilead by winning at the ballot box.  A fundamentalist Christian revolt could happen in the US -- I'm not saying it will -- and one of the precursors would be the Christian right's perception that it is losing ground.  That's when it's time to go for broke.