I came across a phrase, senators who become a scarlet letter, in an article of today’s Washington Post (Jan.29) titled "Tea Party Caucus holds first meeting without some who had embraced banner."
The sentence in question reads as follows:
The Republican senators who rode the tea party wave to victory in the fall are now weighing whether that label will help them on Capitol Hill or become a scarlet letter.
I understand scarlet letter means the letter of A in red signifying adulterer, and it was used as figurative expression by all means. However in this case, I'm puzzled with whom (i.e. political group, faction, clique) do the Senators in question commit adultery, and who is their due (should-be) partner (husband or wife, i.e electral franchise or belonging clique)? Why can they be dabbed a scarlet letter? I read the whole article, but I couldn't figure its meaning out to the end. Can anybody explain?