In 1850, May was sent by President
Franklin Pierce to
Mexico to investigate claims under the United States' treaty of peace with Mexico. He moved to
Baltimore, Maryland. In 1852, May was elected as a
Democrat to the
Thirty-third Congress, serving one term from March 4, 1853 to March 3, 1855. He was an unsuccessful candidate for reelection in 1854 to the
Thirty-fourth Congress, but was elected as a
Unionist to the
Thirty-seventh Congress, serving from March 4, 1861 to March 3, 1863. May sat in the special session of Congress held in summer 1861 after the outbreak of the
Civil War. In September 1861 May was arrested without charges or recourse to
habeas corpus on suspicion of treason and held in
Fort Lafayette. (Lincoln had unilaterally suspended habeas in Maryland in spring 1861, a move ruled unconstitutional without Congressional authorization in June 1861 by Supreme Court Chief Justice
Roger Taney in
ex parte Merryman, a disputed ruling which Lincoln disregarded.) May was eventually released—no charges were ever brought or evidence produced—and returned to his seat in Congress in December 1861. In March 1862 he introduced a bill requiring the federal government to either indict by grand jury or release all other "political prisoners" held indefinitely without recourse to habeas. The provisions of May's bill were included in the
March 1863 Habeas Corpus Act in which Congress finally authorized Lincoln to suspend habeas corpus, but required actual indictments for suspected traitors. The "political prisoners" affected included Baltimore newspaper editor, and vocal Lincoln critic,
Frank Key Howard, who had been a co-prisoner with May, and was also a grand-nephew of Chief Justice Taney's wife Anne Key, (
Francis Scott Key's sister). In 1862, Henry May and Ohio Congressman
Clement Vallandigham, an
anti-war Democrat, led an investigation into telegraphic censorship of the press instituted by Lincoln's Secretary of State
William H. Seward in certain cities. ==Personal life==