CLEVELAND — As Hillary Clinton prepares to announce her running mate later this week, the political calculations have suddenly changed.

Had Donald J. Trump chosen a woman or a minority as his vice-presidential candidate, Mrs. Clinton would have faced pressure from liberals to make her own bold choice. If Mr. Trump had picked a fiery raconteur like Newt Gingrich or Gov. Chris Christie of New Jersey, two of his finalists, the decision would have given Mrs. Clinton pause about putting a young fresh face on the debate stage this fall — rather than an attack dog, according to several Clinton campaign advisers. And if the Republican No. 2 was from a critical swing state, the electoral map might have loomed larger in her considerations.

Instead, Mr. Trump picked Gov. Mike Pence of Indiana — a button-down, deeply conservative white man from a Republican-leaning state whose record appeals strongly to his party’s base.

By choosing someone so conventional, Mr. Trump has made it easier for Mrs. Clinton to follow suit with an unadventurous pick of her own, like Senator Tim Kaine of Virginia or Agriculture Secretary Tom Vilsack, a former governor of Iowa. She could send Housing Secretary Julián Castro out to debate Mr. Pence without fearing a blowout. Or she could pick Labor Secretary Thomas Perez, who is from the reliably Democratic state of Maryland.