Senator Chuck Schumer, the minority leader, has spent months trying to recruit Mr. Hickenlooper to enter the contest to face Mr. Gardner.
Mr. Hickenlooper has pooh-poohed his interest in running for the Senate.
In February he told reporters in Iowa that he is “not cut out to be a senator.”.
Mr. Gardner is seeking a second term in office but faces headwinds in a state where President Trump is highly unpopular.
Democrats must win a net of three Republican-held Senate seats and win the White House to take control of the Senate.
Mr. Hickenlooper in recent weeks has seen an exodus of senior campaign aides, who advised him in the spring to end his presidential bid if his fund-raising and polling status did not improve by the end of July.
Mr. Johnston and Mr. Baer each raised more campaign funds for their Senate campaigns in the second quarter than Mr. Hickenlooper did for his presidential bid. »