Tuesday's primaries may well turn out to have been the decisive point in this year's presidential nominating contests. With wins in Florida, Illinois and North Carolina for Republican Donald Trump and Democrat Hillary Clinton, plus Ohio for Ms. Clinton and neck-and-neck races for both in Missouri, they have, by most estimates, the inside track on amassing the number of delegates they need before this summer's party conventions. Mr. Trump needs only to keep doing as well as he is doing now to amass a majority of delegates and avoid a brokered convention. Ms. Clinton would need a real collapse at this point — a complete reversal of fortunes with Vermont Sen. Bernie Sanders — in order to be stopped.

But for the sake of the candidates, the parties and the nation, we hope both races continue. Florida Sen. Marco Rubio's departure from the Republican contest was expected and necessary, and it leaves Texas Sen. Ted Cruz and Ohio Gov. John Kasich as a well-matched pair of antagonists for Mr. Trump in the months ahead. Meanwhile, Senator Sanders' performance so far has displayed weaknesses in Ms. Clinton's appeal to key segments of the Democratic base, and she needs the time in the spotlight the primaries afford to begin to rectify that before the general election. Mr. Sanders has the money and, with an upset in Michigan, the possibility of victory in Missouri and a string of what the Clinton campaign acknowledges are strong states for him ahead, the rationale to soldier on.

Many Republican leaders, rightly wary of where Mr. Trump is taking their party, have wished for a winnowing of the field of candidates to set up a clear Trump/not Trump choice for voters. The combination of Senator Cruz and Governor Kasich may afford an even better option since the two overlap so little in their appeal. Mr. Cruz is a conservative purist in the tea party mold whose chief appeal to many voters is the very fact that he is so universally disliked in Washington, even by members of his own party. Mr. Kasich is a pragmatist and a moderate (at least by current GOP standards) who can reach out to an increasingly diverse electorate.

Mr. Cruz is likely to be strong in states where Mr. Kasich is weak, and vice versa. Tuesday, for example, Mr. Cruz battled closely with Mr. Trump in Missouri and North Carolina, states where Mr. Kasich barely registered double digits, whereas Mr. Kasich won Ohio handily while Mr. Cruz languished. The combination could be especially crucial in California, the most significant remaining state if Mr. Trump is to be stopped from a delegate majority. Either one alone in that state might not be able to siphon enough support away from Mr. Trump to deny him a win, but together they could chip away at him from both sides.

The ugliness of the Trump phenomenon has become clearer and clearer in recent weeks, and we hope and expect that voters who have flirted with supporting him as a plainspoken outsider will think twice about adding fuel to a campaign that has fostered divisiveness and even violence. We need both Mr. Cruz and Mr. Kasich in the race to give them somewhere to go.

As for the Democrats, Ms. Clinton would risk falling off the national stage altogether if Mr. Sanders were to drop out. Having him in the race gives her the opportunity to reach out to younger voters and address the mistrust that many feel toward her. She needs to do that now and not wait for the general election. At a time when the electorate is clamoring for something new, she is not, and it shows in voters' enthusiasm. It's worth noting, for example, that although she took 64 percent of the primary vote in Florida to Mr. Trump's 46 percent, they got almost the exact same number of votes. The turnout is that much higher on the Republican side in what will be a crucial state in November. In losing Ohio, a similarly pivotal general election state, Mr. Trump got more actual votes than Ms. Clinton did in her victory.

Republicans have reason to worry that they are about to nominate a candidate who has hijacked their party. Democrats have reason to worry that they are about to nominate someone who has failed to excite the base and, in many cases, inspire their trust. Now is not the time to unleash them on each other. Both parties need more time to get their houses in order or else an already alienated electorate could face a dispiriting choice this fall.