SPRINGFIELD, Mo.—President Donald Trump called on Congress to approve a steep cut in corporate tax rates and simplify the U.S. tax system, urging bipartisan support for a tax plan congressional Republicans are sketching out.

Mr. Trump’s remarks were a broad-brush opening pitch for the major tax bill that Republicans aim to pass ahead of the 2018 midterm elections.

Republicans have support from U.S. corporations and a desire to rack up a major legislative victory, but they have little backing from Democrats and must grapple with the competing factions and interest groups that have bedeviled previous tax proposals.

Mr. Trump, a Republican, said he wants to lower business-tax rates to 15% and make it easier for companies to repatriate foreign profits, saying the moves will boost the middle class. But he offered few other details in a 35-minute speech that laid out general principles and flaws in the current system.

The president outlined broad goals to make tax filing easier and lower tax rates on corporations and individuals. He also said he wants to eliminate loopholes that benefit the wealthiest in the nation, though he hasn’t offered many specifics beyond repealing the deduction for state and local taxes.