And in an apparent swipe at Texas Sen. Ted Cruz and GOP front-runner Donald Trump -- with whom the paper has feuded for months -- the Register said voting for Rubio is a choice for "optimism" instead of "anger, pessimism and fear."
The paper had several questions for Rubio, including whether he can make "hard choices" on entitlements, offer a better alternative to Obamacare and clarify his views on climate change. The editorial board also said Rubio has recently "pandered to rising pessimism in his party."
"We hope Marco Rubio and his party take a different path, one that can lead to the opportunity and optimism he so eloquently articulates," the paper wrote.
For Clinton, the paper said in its endorsement of the former secretary of state that she is a "thoughtful, hardworking public servant who has earned the respect of leaders at home and abroad. She stands ready to take on the most demanding job in the world."
The paper added, however, that Clinton is "not a perfect candidate," citing the scandal over her use of a private email server and what it said was a refusal to acknowledge mistakes.
"Her changing stance on gay marriage, immigration and other issues has invited accusations that she is guided less by personal conviction than by political calculations," the paper wrote. "She refutes that, and argues persuasively that a willingness to change one's thinking on specific issues, while remaining true to what she calls 'the same values and principles,' is a virtue, one lacking in most politicians."
Rubio's campaign did not immediately issue a public response to the Register's support. When a voter congratulated him Saturday night at a campaign event in Indianola, Iowa, Rubio responded, "Would rather have your endorsement. Still need you to caucus for me."
Clinton, however, told CNN after a campaign event in Davenport, Iowa, that she was "very pleased" to receive the endorsement.
"Obviously, it means a lot to me," Clinton said.
