Republican Senator Susan Collins of Maine said on Wednesday that she's open to using a subpoena to investigate the tax returns of President Donald Trump in order to ascertain potential connections to Russia. Collins is on the Senate Intelligence Committee which is looking into Russian interference in the election last year. Collins said on Maine public radio that she would hope for "voluntary cooperation" but would be open to the subpoena if necessary. She also said that she would be calling for former national security adviser Michael Flynn to testify. Trump has refused to disclose his tax returns throughout the election.