U.S. Sen. Daniel K. Inouye has formally thrown his support behind Honolulu mayoral candidate Kirk Caldwell.

Caldwell is facing former Gov. Ben Cayetano for the top seat at Honolulu Hale.

Caldwell was an aide to Inouye in the 1970s.

Inouye and Cayetano, two one-time Democratic allies when the latter was governor, have exchanged heated barbs over Cayetano’s opposition to the city’s $5.26 billion rail project.

Inouye, in a statement, said Cayetano’s $1.5 billion alternative Flexible Affordable Smart Transportation plan, would only create more congestion on Oahu.

Cayetano’s election would lead to the loss of $1.55 billion in federal transit dollars, Inouye said in a release.

Cayetano rejects that assertion.