Mr. Rubio “is a powerful speaker,” Ms. Doyle added. “He is young. He is very motivational. He has a powerful story.”

Recognizing how essential it is to win Hispanic support, Mrs. Clinton has gone further in laying out an immigration policy than she has on almost any other issue, saying that she would extend greater protections to halt deportations of people in the United States illegally. She has also hired a former undocumented immigrant to lead her Latino outreach efforts.

Her own strategists, their allies in the “super PACs” working on her behalf and the Democratic Party all say they see plenty of vulnerabilities in Mr. Rubio’s record and his views. And they are trying to shape the perception people have of him while polls show that he is still relatively unknown: Yes, the Democratic National Committee said in a recent memo, Mr. Rubio was a fresh face, but one “peddling a tired playbook of policies that endanger our country, hurt the middle class, and stifle the American dream.”

So far, Democrats who have combed over Mr. Rubio’s voting record in the Senate have seized on his opposition to legislation raising the minimum wage and to expanding college loan refinancing, trying to cast him as no different from other Republicans.

The subtext: He may be Hispanic, but he is not on the side of Hispanics when it comes to the issues they care about.

Democrats will try to use Mr. Rubio’s youth and four-year career in national politics against him, depicting him as green or naïve — a liability at a time when unrest abroad is a top concern. “A Dan Quayle without the experience,” suggested Christopher Lehane, a veteran strategist who has worked for the Clintons.