Ten Eyck served in the U.S. Senate from March 4, 1859 to March 3, 1865, after winning election in a joint session of the New Jersey State Legislature which met in January 1859. The anti-slavery Opposition Party, which consisted of members of the new Republican Party, traditional Whigs, members of the Free Soil Party, and members of the American Party vied with Democrats for control of the legislature and selection of a U.S. senator. With none of the Opposition groups strong enough to elect a candidate on their own, but determined to prevent the re-election of William Wright or the election of another Democrat, the Opposition eventually decided to agree on a compromise candidate who had no strong ties to any faction. They selected Ten Eyck, who was not an active candidate, but was known to have been a Whig, and more recently a Republican, yet not politically prominent in recent years or strongly committed to any Opposition faction. Members of the American Party were especially unhappy at being unable to elect John F. Randolph or another American candidate, but accepted Ten Eyck to ensure that a Democrat would not win the seat. During his Senate career, which spanned the American Civil War, Ten Eyck served on the Judiciary and Commerce committees. He entered the Senate as a presumed moderate, and opposed allowing slavery to expand, but believed the Constitution permitted it where it existed. He also supported enforcement of the Fugitive Slave Act of 1850 even though he was personally opposed to it, on the grounds that he believed it was constitutional. Over time, his anti-slavery views became more pronounced. When the New Jersey State Legislature passed early 1861 resolutions in support of the Crittenden Compromise, Ten Eyck complied with the legislature's instructions to transmit them to the Senate, but made clear that he believed they were unconstitutional. Ten Eyck voted to end slavery in Washington, D.C., and voted in favor of the Thirteenth Amendment, which abolished slavery in the United States. When soldiers of the Confederate States Army invaded Pennsylvania in the summer of 1863, Ten Eyck advocated immediate creation of a Burlington County militia unit to take part in Pennsylvania's defense. He enlisted as a private in order to set an example for those in attendance, and they called the unit they created the "Ten Eyck Guards" in his honor. He marched to Pennsylvania with the company, and once the Confederates had retreated, he completed his term of service upon the return of the Ten Eyck Guards to Burlington County.
Post-Senate career
After leaving the Senate, Ten Eyck resumed practicing law. He was a delegate to the 1866 National Union Convention, which attempted unsuccessfully to promote post-Civil War reconciliation, including unity behind the Reconstruction policies of President Andrew Johnson. In 1873, he was appointed to the commission that revised the Constitution of New Jersey, and served as its president following the death his predecessor. The commission submitted its proposed changes to the state legislature, who presented to them voters for ratification. The changes were approved in an 1875 election, and went into effect soon afterwards.
Death and burial
Ten Eyck died at his home in Mount Holly Township, New Jersey on August 24, 1879. and was interred in Mount Holly's St. Andrew's Cemetery.
Family
On June 10, 1845, Ten Eyck married Julia Gadsby, the daughter of John Gadsby and Providence Gadsby. They were the parents of six children -- Augusta, Julia, Jane, May, Virginia, and John.
