WASHINGTON — Speaker Paul D. Ryan of Wisconsin drew a striking contrast between Republicans and Democrats on Thursday and vowed to pursue legislation that would frame a stark choice for voters in 2016.

“Our No. 1 goal for the next year is to put together a complete alternative to the left’s agenda,” Mr. Ryan said, speaking in the Great Hall at the Library of Congress, a setting designed to highlight the importance he placed on the speech. “Only government that sends power back to the people can make America confident again.”

With the speech’s sweeping oratory and careful stagecraft, it was clear Mr. Ryan was aiming to step decisively into the role of the Republican Party’s leader in Washington, and to set himself apart not just ideologically from Democrats but also in tone and substance from some of the recent coarse language of his own party’s presidential candidates.

But while Mr. Ryan is now the senior Republican in government — second in presidential succession only to the vice president — he has yet to demonstrate that rank-and-file lawmakers will follow his lead, let alone candidates like Donald J. Trump and Senator Ted Cruz, whose campaigns are rooted in anti-establishment sentiment.