On her way to a second post office, she found a petitioner at a grocery store urging customers to exercise their civic duty. But the petitioner had run out of forms, too.
The second post office said it had not had any forms in stock for a week. Refusing to give up, Ms Tidwell tried the public library. A librarian told her they had been requesting forms for over a month, without any luck.
Finally Ms Tidwell drove to the county registrar's office in the distant suburb of Norwalk to make sure she got on the electoral list. "I had the car and almost two hours to invest in this but what about potential voters who don't?" she said.
It has become a cliche of American political life that the single largest constituency in any election are the non-voters. Turn-out has been steadily declining since the Second World War and now barely reaches 50 per cent, even for the presidential contest. In a non-presidential year like this one, there is a good chance that turn-out will hit a record low.
Of all the reasons why Americans do not vote - disillusionment with the political system, a sense of disenfranchisement, straightforward apathy - one significant factor is the sheer mechanical difficulty of going through with the whole thing.
Officials might offer extenuating circumstances to explain Ms Tidwell's difficulties - the urban dystopia that is Los Angeles, the fact that she waited until the last registration day, a regrettable degree of bad luck - but her sense of frustration is shared far and wide.
In California alone, 5 million eligible voters (roughly a quarter of the total) are unregistered. Although registration forms are now more widely distributed than they used to be, picking one up often involves long queues at the post office or the Department of Motor Vehicles.
And registration is only the beginning of the story. Voting itself is a time-consuming process, partly because there are so many election races to pronounce on, and partly because polling stations are relatively scarce, making the shoddy turn-out something of a self-fulfilling prophecy. There is no way of knowing if you will be through in five minutes, or two hours.
Given that election day always falls on a Tuesday, people with a busy work schedule, or a less than understanding boss, have to queue ahead of the 7 o'clock opening time, or else join the after-work rush. (The polls close at 8pm, a squeeze for many in such a work-driven society.)
Then there is the voting form, which is not a simple sheet but a bulging booklet. Picking a candidate for governor or senator may be straightforward. Parents are likely to have strong feelings about the local school board. But how is the average Joe supposed to have informed opinions on all these, plus the nominations for superior court judge, the county commission and the police department?
And what about all those local ballot initiatives - on everything from abortion rights to the debt financing of public utility companies - that can require a master's degree in public administration to understand?
It is a case of what the New York Times columnist Maureen Dowd recently called TMI - too much information, way too much information. It may look like democracy in action, but half the country has given up before they have even finished reading the ballot.Reuse content