Sen. Chuck Grassley Charles (Chuck) Ernest GrassleyGOP lawmakers distance themselves from Trump comments on transfer of power The Hill's 12:30 Report: Ginsburg lies in repose Top GOP senators say Hunter Biden's work 'cast a shadow' over Obama Ukraine policy MORE (R-Iowa) encouraged President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE to refrain from personal attacks on his opponents and to instead target their policies.

"Thought long time how Trump can improve his approval rating majority of the people like his policies stick to policy don’t attack individuals but state why opponents policy wrong," Grassley tweeted Thursday before the second night of the first Democratic debates.

"Don’t denigrate them personally if somebody is stupid bite your lip," he added.

Thought long time how Trump can improve his approval rating majority of the people like his policies stick to policy don’t attack individuals but state why opponents policy wrong don’t denigrate them personally if somebody is stupid bite your lip — ChuckGrassley (@ChuckGrassley) June 27, 2019

Trump is known for his personal jabs at political opponents, including labeling them with nicknames.

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About a half-hour into Wednesday night's debate, Trump tweeted

"BORING!" He also criticized NBC News over a technical issue that led to an extra commercial break.

Grassley last month criticized the president over his proposed tariff on Mexico and last week expressed opposition to a Trump proposal aimed at lowering drug prices.