The SEC is being sued by two groups to block this.
"The S.E.C., in a 3-2 vote, stipulated that investors or groups of shareholders who have owned 3 percent of a company for three years could have candidates on proxy statements. Under the regulation, shareholders would be able to nominate at least one director and as much as 25 percent of a board. "
We will talk more about it in class, but I have a hard time imagining a firm being "hijacked" by the inclusion of another candidate. Shareholders do not need to vote for the candidate just because (s)he is listed on the proxy.