“If a person wants to be the nominee of the Republican Party, there can be no evasion and no games,” House Speaker Paul Ryan said on Tuesday, after months of such games. He sounded naïvely unaware of the darker elements within the Republican Party, present for decades, and now holding sway: “This party does not prey on people’s prejudices. We appeal to their highest ideals. This is the party of Lincoln.”

The Republican Party is taking a big step toward becoming the party of Trump. Those who could challenge Mr. Trump — Ted Cruz and Marco Rubio — are not only to the right of Mr. Trump on many issues, but are embracing the same game of exclusion, bigotry and character assassination. That Mr. Rubio would make double entendres about the size of Mr. Trump’s hands and talk about Mr. Trump wetting his pants shows how much his influence has permeated this race and how willingly his rivals are copying his tactics.

There were opportunities to stop this, early on. Possessed of a crack team of researchers, the Republican Party did not turn its resources on investigating this man’s record of falsehoods and business failures. When struggling families worry that their children’s American dream will be obscured behind a mountain of college debt, even a passing reference to the scam that is Trump University would surely have resonated many months ago. It may now be too late to alter the course Mr. Trump has set, but Republican leaders would be derelict not to try.

On the Democratic side, Mrs. Clinton, who is pulling away in the contest with Bernie Sanders, should stick to the high ground. Say what one will about Mr. Sanders’s bid, he has run a campaign admirable in its restraint and positive ideals, no easy feat this year. Mrs. Clinton should continue to campaign on who she is and what she can do for middle-class Americans seeking leadership that reflects the best, not the worst, of political impulses.