Yes, it should be legal for companies to ask employees to vote for a political candidate, because that is simply the company exercising their own free speech. It would be wrong for the employer to take action against an employee or threaten them into voting for a particular candidate. But the employer can ask and give reasons why it's in the company's best interests.
i don't think it should be legal for any company to ask employees to vote for a political candidate, because that easily takes us back to the early to mid-1900's when politics were more openly crooked. this would start a slippery slope where all a politician has to do it buy out companies large enough and make the employees vote for them.