Washington (CNN) Iowa's longest-serving Republican state lawmaker is ditching the party in a protest of what he called President Donald Trump's "unacceptable behavior" and is joining the Democrats.

State Rep. Andy McKean, a moderate from eastern Iowa whose 29 years in the legislature include stints in the House and Senate, announced his party switch at a news conference Tuesday. He called Trump "a poor example for the nation and particularly for our children" and said he'll seek re-election in 2020 as a Democrat.

"With the 2020 president election looming on the horizon, I feel as a Republican that I need to be able to support the standard bearer of our party. Unfortunately, that is not something I am able to do," McKean said Tuesday of Trump.

"He sets, in my opinion, a poor example for the nation and particularly for our children by personally insulting -- often in a crude and juvenile fashion -- those who disagree with him, being a bully at a time when we are attempting to discourage bullying, his frequent disregard for the truth and his willingness to ridicule or marginalize people for their appearance, ethnicity or disability," he said.

The move -- which narrows the GOP's hold on the state House to 53 seats to Democrats' 47 -- is an embarrassment for Trump in a state that's under a constant political microscope due to its role as the first to cast votes in presidential nominating contests. Democratic presidential contenders will battle for the next nine months ahead of the state's caucuses next year.

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