My thought is that the person unable to vote would either say:
I authorize you to fill out a ballot voting for candidate X
I am unable to research the candidates, but you know my values and I authorize you to fill out a ballot voting for whichever candidate best
aligns with them
The former is permissible, the latter is an impermissible proxy.
The choice of who and what you vote for is non-delegable in every U.S. state.1 Proxy voting2 has been prohibited in governmental elections in the U.S. since not long after the U.S. Civil War, although it persisted longer (until the early 1900s) in the internal elections of political parties.
In the case of a severely disabled person, that person may obtain assistance at the polls from an election official, and elsewhere, from someone else of the voter's choice, with an absentee ballot, to actually mark the ballot to the disabled person's precise direction.
Now, in theory, the precise direction could be succinct, if the voter's intentions are clear and simple.
For example, the voter might direct someone to "vote a straight party-line ballot for partisan candidates for the GOP, vote against retaining each judge facing a retention election, vote in favor of all referred ballot issues, and vote against all citizen initiatives."
Assistance in marking a ballot is permitted even in states where absolute secrecy in ballot completion (which cannot be waived even by the voter) is mandated by law and disclosure of a marked ballot to third-parties is otherwise prohibited by law. Indeed, this is required by the Americans With Disabilities Act and related laws and case law (some general principles are laid out here). As the U.S. Department of Justice explains:
To ensure that voters with disabilities can fully participate in the
election process, officials must provide appropriate auxiliary aids
and services at each stage of the process, from registering to vote to
casting a ballot. Only if providing an aid or service would result in
a fundamental alteration or undue financial and administrative burdens
is a jurisdiction not required to provide the aid or service.
However, the jurisdiction still has an obligation to provide, if
possible, another aid or service that results in effective
communication. In determining the type of auxiliary aid and service
to be provided, officials must give primary consideration to the
request of the voter.
Examples of auxiliary aids and services for people who are blind or
have low vision include a qualified reader (a person who is able to
read effectively, accurately, and impartially using necessary
specialized vocabulary); information in large print or Braille;
accessible electronic information and information technology; and
audio recording of printed information.
Examples of auxiliary aids and services for people who are deaf or
have hearing loss include sign language interpreters, Video Remote
Interpreting, captioning, and written notes. For additional
information about auxiliary aids and services, see ADA Requirements:
Effective Communication at http://www.ada.gov/effective-comm.htm.
For example, suppose that a jurisdiction is conducting an election for
mayor and city council members using a paper ballot system. A blind
voter requests an accessible ballot. A Braille ballot would have to
be counted separately and would be readily identifiable, and thus
would not constitute a secret ballot. Other aids and services would
better afford voters who are blind the opportunity to vote privately
and independently and to cast a secret ballot, just like other voters.
These may include ballot overlays or templates, electronic information
and information technology that is accessible (either independently or
through assistive technology such as screen readers), or recorded text
or telephone voting systems.
But, the only relief available to someone "unable to focus for a sustained period of time", is to get an absentee or mail-in ballot and to do a little bit at a time.
The law of what is permissible in terms of someone strongly suggesting someone vote a particular way, particularly if they can see how someone is actually voting, is less uniform.
The U.S. Tax Code prohibits 501(c)(3) exempt organizations from endorsing candidates. Many states have laws prohibiting campaigning by employers directed at workers. Campaigning in government workplaces is likewise banned in many places and circumstances.
1 There is a very narrow exception allowing voting by proxy in some states when one is voting because one is a property owner, who may or may not be required to be natural person depending upon the state, in a special district election. This can happen in special districts where there are no residents, or where state law otherwise allows property owners to impose taxes upon themselves collectively in a special district to provide services. For example, property owners may vote by proxy in a water and sewer district bond levy election in an industrial park in many states.