Ohiomeister--If I had to guess, I'd say you were exactly right. On the last race I worked (which happened to be the most expensive House race in history when all was said and done), we would sit someone with a laptop set to our ActBlue page next to the candidate when she did call time, and just directly input credit card contributions by collecting the information over the phone. It's cheaper and much more reliable than getting a pledge for a check, since you collect the contribution instantaneously, rather than wait for someone to sign over a massive sum of money that they'd probably rather be spending on something tangible. The fewer opportunities someone gets to change their minds about giving, the better.

(Incidentally, while we're all versed in the arguments about how, in the end, endorsements don't really make that much of a difference--they ALWAYS make a difference if they come with donor databases. A Presidential candidate can do a lot of damage with a proven House donor database and, as I recall, Mitt's leading in the endorsements column.)