The Senate postponed a vote on former Republican Sen. Chuck Hagel's nomination for secretary of defense after Republicans raised questions about who paid for some of his speeches and consulting work.

Lawmakers want to know if Hagel got any money directly or indirectly from foreign sources. They also want to know why Hagel has refused to answer those questions.

"This committee, and the American people, have a right to know if a nominee for secretary of defense has received compensation, directly or indirectly, from foreign sources," 25 senators said in a letter to Hagel Tuesday.

"Your refusal to respond to this reasonable request suggests either a lack of respect for the Senate's responsibility to advise and consent or that you are for some reason unwilling to allow this financial disclosure to come to light," the letter reads.

The senators say they cannot vote on his nomination until they have all the information they need.