Donald J. Trump’s series of victories on March 15 extended his delegate lead and forced Senator Marco Rubio of Florida out of the presidential race. Mr. Trump’s path to winning enough delegates to secure the Republican nomination is not assured, but he is in a strong position.

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If Mr. Trump maintains his current level of support in the remaining races, he is almost certain to secure the nomination.

After the contests on March 15, no other candidate retains a real chance of capturing the delegates required to win the nomination outright. Mr. Rubio dropped out, Gov. John Kasich of Ohio is too far behind, and Senator Ted Cruz of Texas would need to win the vast majority of the remaining delegates — a near impossibility.

But Mr. Trump still needs to win most of the remaining delegates to avoid a contested convention.

If he continues his current performance and wins a series of key states — like California and New York — he would get the needed delegates.

Mr. Trump will probably need to win California, which has 172 delegates. California is winnable for Mr. Trump, but it could be a difficult state for him. California includes a mix of well-educated voters who could support Mr. Kasich and conservative voters who could support Mr. Cruz.

If Mr. Trump loses California, he could miss the delegate cutoff.

Exit polls have indicated that most of Mr. Rubio’s support could be distributed to Mr. Trump’s competitors. Say 80 percent of Mr. Rubio's voters go to Mr. Cruz. This would cut into Mr. Trump’s delegate lead.

But even that may not prevent Mr. Trump from winning the key states — like California — that ensure him enough delegates.