Formal:
* Newly elected official: This is a general term that applies to any elected position.
* Elected Senator: This is the most straightforward option, but it doesn't emphasize the newness.
* Freshman Senator: This term specifically denotes a first-term Senator.
Informal:
* Newbie: This is a casual term, often used in a humorous way.
* Rookie: Similar to "newbie," this emphasizes the lack of experience.
* First-timer: This is a neutral way of saying someone is in their first term.
The best choice will depend on the specific situation and the tone you are trying to convey.