Among the topics that Republican presidential nominee Donald Trump is expected to raise when he meets with evangelical Christian pastors today in Orlando: the “Johnson Amendment.”

Trump has repeatedly vowed to “get rid of” the tax rule that Lyndon B. Johnson introduced in 1954 while running for reelection as a Texas senator -- a provision that some religious groups and churches consider an infringement on their freedom of speech.

“I am going to work very hard to repeal that language and to protect free speech for all Americans,” Trump said during his speech at the National Republican Convention in Cleveland last month as the crowd erupted in applause.

The rule bars tax-exempt charitable groups, which the Internal Revenue Service calls “section 501(c)(3) organizations,” from endorsing candidates and getting involved in political campaigns. Because churches fall under section 501(c)(3) of the tax code, the ban applies to them as well.

“Under the Internal Revenue Code, all section 501(c)(3) organizations are absolutely prohibited from directly or indirectly participating in, or intervening in, any political campaign on behalf of (or in opposition to) any candidate for elective public office,” according to the tax code.

Trump blames Johnson for “silencing the church.” That’s not quite what LBJ intended. What was he trying to do? Here's a rundown.