act as a witness

affirm assert asseverate assure avert avow back certify confirm contend corroborate cosign declare guarantee maintain okay predicate profess prove sponsor substantiate support testify uphold verify vow warrant witness

answer for attest to be responsible for bear testimony get behind give an affidavit put forth rubber-stamp say so sign for stand up for swear to swear up and down

break condemn contradict deny desert disapprove disavow disclaim discourage discredit disprove invalidate neglect oppose refuse refute reject repudiate veto

renounce