WASHINGTON — She holds a commanding lead in the presidential race and has $150 million in the bank, but some leading Democrats are urging Hillary Clinton to use more of that money to mobilize black voters in hopes of winning control of Congress, publicly revealing a crack in party unity that could have lasting consequences if she is elected.

In interviews, two senior African-American members of the House called on Mrs. Clinton to draw from her war chest to fund voter-turnout efforts in congressional races. They warned that even if she handily defeated Donald J. Trump, her presidential agenda would be stymied unless she swept in new Democratic lawmakers with her.

At issue is a strategic choice with profound implications: Should Mrs. Clinton reach to defeat Mr. Trump in more states like Utah? Or should she instead divert some of her resources to Democrats who are battling in tight races in liberal states like New York and centrist states like Colorado, or in Republican-leaning states like Indiana and Missouri that she has effectively written off?

As Mrs. Clinton confidently expands her campaign into conservative-leaning states, she should make the knife’s-edge fight for the Senate and the Democratic effort to cut into the Republicans’ House majority a priority, said the lawmakers, Representatives James E. Clyburn of South Carolina and G. K. Butterfield of North Carolina.