On the last day to file papers for candidacy, this year’s race for Honolulu mayor is finally coming into focus.

Former U.S. Rep. Charles Djou and former Mayor Peter Carlisle are the seasoned politicians so far who will challenge incumbent Mayor Kirk Caldwell for the city’s top electoral post. Former Lt. Gov. James “Duke” Aiona could also join the mayoral race, or he could run for Congress or the state Senate. Today marks the deadline for Aiona to file papers and enter one of those races.

Djou is slated to formally announce his candidacy today at Honolulu Hale in the nonpartisan race that will likely focus on the rail project, homelessness and the public’s trust in city government.

Regarding rail, Djou, a former congressman, Honolulu councilman and state representative, said Monday, “As I look around, no one is taking responsibility for the incompetence, mismanagement and breakdown of this project.” The project faces a more than $1 billion budget shortfall despite a recent five-year tax extension to complete the entire 20-mile elevated line. “No one is taking responsibility, so I will,” he said.

Djou said that he would veto any further tax extensions for rail, saying, “I believe we need to make this project work with the money we have.”

However, he added that ending the project at Middle Street would not be acceptable. Rail officials estimated last month that it would cost some $6.8 billion to complete the line — “so do it,” Djou said Monday.

Rail’s federal partners estimate it could cost $8.1 billion or more to complete the project. Djou said he would be open to other ideas to finish rail that did not involve additional taxpayer dollars. He did not specify further other than to say he would aim to cut the state’s 10 percent fee to administer rail’s general excise tax surcharge.

Carlisle, who helped advance the rail project before losing his re-election bid in the 2012 primary, said the city must do all it can to get rail all the way to Ala Moana Center.

“You do everything humanly possible to keep it on budget, which may not be possible, and you finish the job. You do not stop at Middle Street,” Carlisle said Monday. “The cost involved in stopping now and getting a brand-new environmental impact statement and kicking the can down the road another decade are unacceptable.” He filed papers Monday to run after declaring his candidacy several weeks ago.

Both Djou and Carlisle will have a long way to catch up to Caldwell’s campaign account, which totaled more than $1.6 million as of Jan. 1, according to state Campaign Spending Commission filings. In April, while weighing a mayoral run, Carlisle acknowledged that he had no funds in his campaign account. On Monday, Djou said he had not yet raised any funds. Aiona’s filings show he had nearly $18,900 in campaign funds as of Jan. 1.

City Council Chairman Ernie Martin, meanwhile, announced Monday that he won’t join the race for mayor — ending nearly two years of will-he-or-won’t-he speculation.

Martin, who would have needed to resign as the Council’s leader to run for mayor, cited the city’s problems on rail and homelessness as key reasons for his decision to stay put.

“I believe that this is the right course of action given the numerous challenges facing Honolulu,” a statement posted Monday on Martin’s website read. “I have taken the lead in ensuring that the rail project is accountable to how it spends our money and advocated for innovative and community-based solutions in addressing our homelessness crisis.

“To step aside for my personal ambitions at a time when these issues continue to demand true leadership would be selfish,” it further read.

Martin’s most recent filings with the state Campaign Spending Commission show that he had a campaign war chest of more than $492,000 as of Jan. 1 — funding that he could use in future races. Term limits will prohibit him from running for re-election to the Council in 2018. It remains to be seen what political plans he might have for the future.

Martin did not respond to requests for comment Monday, so it’s not clear whether other factors discouraged him from joining the mayoral race.

Previously, Martin acknowledged that he wanted the job. In August 2014, after cruising handily to Council re-election, Martin posted a photo on social media of himself posing with a large “Ernie Martin Mayor” sign.

Several days before posting that image, when asked whether he might run for mayor, Martin said Caldwell had supported Martin’s opponents in recent years and that “coming in 2016, I’ll be seriously looking to return the favor.”

Caldwell released a statement Monday pertaining to Martin’s decision. It read, “I know it was a difficult decision to make, but I appreciate Chair Martin’s careful consideration and I look forward to working with him over the next two years that he will serve on the Council.”