Walsh recently told me ratings on his syndicated talk show have held about even, with moderate listeners replacing defecting Trump loyalists. But he admitted that he has recently lost a big market, Houston, with some other stations also dropping his show in the wake of his ongoing critiques of the president.

Walsh has been active in Chicago-area politics for some decades, once running as a "moderate" against the late North Side U.S. Rep. Sidney Yates. But he moved to the right when he moved to the suburbs and captured a seat there, burnishing his conservative bona fides with near-constant attacks on then-President Barack Obama, insisting he would support tough border legislation even if it involved building moats filled with alligators, and calling for "profiling" young male Muslims. After his district was remapped by Democrats after the 2010 Census, Walsh lost his seat to Tammy Duckworth, now a U.S. senator.

Trump is not likely to take any challenge lightly—particularly from an outspoken radio commentator who has said and done lots of provocative things during his career and whose personal biography includes a high-profile child-support dispute with his ex-wife, which was settled in 2012.

One other nationally known Republican already is running, former Massachusetts Gov. Bill Weld. But he is a social moderate in a distinctly conservative party and ran in 2016 as the Libertarian Party candidate for vice president. Others reportedly considering a run against Trump include former South Carolina congressman Mark Sanford and former Ohio Gov. John Kasich.

Here's a quick roundup of some of the Twitter reaction to the Walsh news: