Harkin said that he adopted his view "a long time ago" while leading a congressional committee that oversaw the District.
His comments came during a press conference celebrating new preventive care benefits available to women under the Affordable Care Act.
Franks's bill would criminalize abortions in D.C. after 20 weeks of pregnancy because, Republicans say, some research suggests fetuses feel pain at that point. Doctors who disobey the law would be fined or imprisoned for up to two years.
The bill also contains new reporting requirements and, under certain circumstances, civil remedies for partners and parents of women who have abortions. It would not exempt for victims of rape or incest, or for women whose pregnancies threaten their health.
Groups that support the bill argue that D.C.'s liberal abortion policy needs limits.
Opponents, which include advocates for D.C. home rule, say the bill "doubly" violates the Constitution by contradicting Roe v. Wade and singling out only some Americans.
The bill will come to the floor Tuesday under suspension of the rules, which sets a two-thirds vote hurdle for passage and likely dooms the measure to failure.