How far are priests and other religious allowed to go in terms of political endorsements (or non-endorsements)? In the USA, I've heard priests say that they can't endorse any candidates but the Bishops put out a fairly unambiguous voters guide every election that clearly says there are non-negotiable issues (mainly concerning life and marriage) which align themselves squarely with the more conservative parties.
But Fr. James Altman of the diocese La Crosse in Wisconsin is of the mind that &quot;You can't be a Catholic and a Democrat. Period&quot; and made it known on a widely circulated youtube video. 
Possibly in response to this, a pastor at a local parish posted on Facebook:

The Catholic Church does not endorse a political candidate or a party. This means, that as a priest, I cannot endorse anyone.

and Fr. Altman seems to be in a bit of hot water from his Bishop for his statements.

However, why can't a priest tell a person not to vote for someone.  If a Catholic is running for office who is excommunicated, wouldn't that be a non-endorsement of that person's character by the Church and if a person is running for office who is not excommunicated, but probably deserving of that excommunication, does that mean the Church cannot excommunicate that person for fear of looking like a political endorsement for his or her rival?
The Johnson Amendment is a provision in the U.S. tax code, since 1954, that prohibits all 501(c)(3) non-profit organizations from endorsing or opposing political candidates. Section 501(c)(3) organizations are the most common type of nonprofit organization in the United States, ranging from charitable foundations to universities and churches. The amendment is named for then-Senator Lyndon B. Johnson of Texas, who introduced it in a preliminary draft of the law in July 1954.
The Internal Revenue Code prohibition against political campaign intervention applies to religious organizations as tax-exempt organizations and to the actions of clergy or other religious leaders acting as representatives of their religious organizations. The prohibition does not apply to the political activities of clergy or other religious leaders undertaken in their individual capacities. Thus, clergy or other religious leaders, in their individual capacities and outside the context of any religious
organization-sponsored function or publication, may endorse or oppose candidates and otherwise participate in election campaigns. When engaging in personal political activities, however, clergy and religious leaders should take steps to ensure that their actions are not imputed to their religious organizations.
While the amendment states that the organization must be organized and operated exclusively for religious, educational, scientific or other charitable purposes, only churches are automatically considered exempt and are not required to apply for and obtain recognition of tax-exempt status from the IRS, although religious organizations are not treated more harshly than schools, hospitals, social services agencies, colleges and universities, scientific organizations, museums or other charitable organizations exempt under section 501(c)(3) of the code.4 None of these organizations may intervene in political campaigns.
The First Amendment provides that “Congress shall make no law respecting an establishment of religion or prohibiting the free exercise thereof ...” Although the Internal Revenue Code prohibition against political campaign intervention may burden the exercise of religion to the extent that a religious organization must choose between the receipt of the benefits of tax exemption and intervention in a political campaign, not every burden on religious exercise is constitutionally prohibited.
To date, courts have been unsympathetic to First Amendment challenges to the political campaign intervention prohibition. In the most prominent ruling on this prohibition, the Court of Appeals for the D.C. Circuit upheld in 2000 the constitutionality of the political campaign intervention prohibition as applied to a church, concluding that the prohibition did not violate either the Establishment Clause or the Free Exercise Clause of the First Amendment.
This prohibition encompasses a wide array of activities. It precludes direct
political campaign intervention, including the making of statements, whether oral, written or in an electronic medium, supporting or opposing any candidate, political party or political action committee (“PAC”); creating a PAC;8 rating candidates;9 and providing or soliciting financial support (including loans10 or loan guarantees) or in-kind support for any candidate, political party or PAC. It also precludes indirect political campaign intervention of a sort that reflects bias for or against any candidate, political party or PAC, such as distributing biased voter education materials or conducting a biased candidate forum or voter registration drive.
The political campaign intervention prohibition does not restrict discussions of issues that are not linked to support for or opposition to candidates. The fact that candidates may align themselves on one side or another of an issue does not restrict the ability of religious organizations to engage in discussions of that issue. That said, a religious organization may nonetheless violate the political campaign intervention prohibition if it communicates preferences for or against particular candidates as part of its issue discussions.
For a more exhaustive document on all aspects of politics and religion, see this Pew research paper.