In 1621 Seymour was elected
Member of Parliament for
Wiltshire. In May he proposed that distinctly harsh penalties should be inflicted on Edward Floyd. He was elected MP for
Marlborough in 1624. In that parliament he worked hard to induce a war with Spain, but protested against any extensive military operations in continental Europe and opposed sending an army to the Palatinate on the ground of the "extreme charge". In 1625 he was again elected MP for Wiltshire. On 30 July he proposed that the grant be limited to one subsidy and one-fifteenth, about a tenth of what King Charles I required to meet his engagements. He rejected the overtures which the Duke of Buckingham made to him, and in July he refused to join in the attack on Lord-Keeper Williams because Buckingham was secretly abetting this. In August he attacked the government for conducting a continental war, inveighing against peculation in high places and the sale of offices at court. On these grounds he dissuaded the House from granting supplies. He was re-elected to the new parliament summoned in February 1626, but was made
Sheriff of Wiltshire to prevent his sitting. In the following July his name was struck off the commission of the peace. Thenceforth Seymour adhered to Wentworth's policy of moderation. In 1628 he was elected MP for Wiltshire and Marlborough and chose to sit for Wiltshire. On 29 April he joined Noy and Digges when they tried to modify the Commons' Bill of Liberties, and supported Wentworth's
Habeas Corpus Bill. He also advocated, with Wentworth against Eliot, a joint-committee of the two Houses on the Petition of Right. In May 1639 Seymour refused to pay
ship-money, and in the following March he was elected without opposition as MP for Wiltshire to the
Short Parliament. In this parliament he spoke powerfully against granting any subsidies to the King before receiving any redress of grievances, and apparently compared
"our affayres to the bondage of the Israelites in Egypt". In November 1640 he was elected MP for Marlborough to the
Long Parliament. He soon began to differ from the popular party, and on 19 February 1641 he was created
Baron Seymour of Trowbridge, Wiltshire, a year after his elder brother had been created
Marquess of Hertford. In the House of Lords he insisted on voting against
Strafford's attainder, although the opposite party denied his competence to vote because he was not a peer when the charges against Strafford were first brought up. ==Civil War==