Steve Schmidt served as a top campaign adviser to George W. Bush in 2004 and John McCain in 2008.

For months, Steve Schmidt, a longtime Republican strategist, has warned about the degradation of his party, saying the Trump administration is responsible for a “coarsening of this country” and calling the president a “useful idiot” for Russia.

Now, Mr. Schmidt says he’s done: On Wednesday morning, he renounced his membership in the Republican Party, nearly three decades after joining it, and called for a Democratic wave in the midterm elections this fall.

29 years and nine months ago I registered to vote and became a member of The Republican Party which was founded in 1854 to oppose slavery and stand for the dignity of human life. Today I renounce my membership in the Republican Party. It is fully the party of Trump.

— Steve Schmidt (@SteveSchmidtSES) June 20, 2018

In a series of tweets, Mr. Schmidt, who served as a top campaign adviser to George W. Bush in 2004 and John McCain in 2008, said that the party he long served had become “corrupt, indecent and immoral.” He pointed to the Trump administration’s practice of separating children from their parents when apprehended at the…