Among the Republican Party’s announced and expected candidates, Mr. Rubio occupies a middle ground, which is both an asset and an obstacle. He hopes to appeal to more moderate Republicans as well as to social, fiscal and foreign policy voters, but he could also find himself without a clear constituency, especially in the first four nominating states.

Mr. Rubio has credibility with the conservative grass roots after defeating both a Democrat and Charlie Crist, a former moderate Republican governor, in his Senate race, but he offers a message that is not as hard-line as those of Republicans like Mr. Cruz and Mr. Walker.

As a member of the Foreign Relations and Intelligence Committees, Mr. Rubio has used his time in the Senate to position himself as a hawk, a stark contrast with Mr. Paul, who prefers a more restrained approach to military intervention. After his announcement here, Mr. Rubio plans to travel back to Washington to attend a Foreign Relations committee meeting on legislation that would require Congress to weigh in on any nuclear deal reached with Iran.

But his work on immigration — one of his biggest achievements in the Senate — illustrates the delicate balance Mr. Rubio will have to strike to make it through his party’s nominating process. In 2013, Mr. Rubio was part of a bipartisan group of senators that drafted a broad immigration bill that included a pathway to citizenship for the 11 million unauthorized immigrants already in the country.

He has since distanced himself from the proposal, saying he believes any immigration overhaul must start with securing the nation’s southern border and proceed step by step. But his original legislation enraged the right, which saw it as amnesty, while many liberals and immigration groups thought he had not gone far enough and were frustrated with his position.