How should a congressperson be addressed in a letter of recommendation?

[SA.applicant98]

I have several people writing letters of recommendation for me for service academy nomination applications. How should they address those letters to the congressperson? I've seen several ways online...
-Dear Senator John Doe,
-To whom it may concern,
-Dear Board of Nominations,
-Dear Honorable John Doe,

I'm not sure if these are eligible, or if there is a specific address I need to be using? Honorable John Doe seems the most respectful.

Any help is appreciated.

 

[MidCakePa]

“Dear Senator Doe” will suffice. (“Honorable” is used when referring to them in the third person.)