Now that Adolfo Carrión, Jr. has officially vacated his position as Bronx borough president, the door is wide open for candidates vying for his seat. But as of Wednesday morning, only one candidate, Assemblyman Ruben Diaz, Jr., has officially thrown his hat in the ring.

On March 2, Mayor Bloomberg announced that a non-partisan special election for Bronx borough president will take place on April 21. According to Marcus Cederqvist, executive director of the city Board of Elections, candidates need to obtain 4,000 petition signatures from registered Bronx voters by March 16 to get on the election ballot.

Assemblyman Ruben Diaz, Jr. has already won the support of the Bronx County Democratic Committee, as well as the Riverdale-based Benjamin Franklin Reform Democratic Club and the Liberty Democratic Association. His likely rival will be longtime colleague, Council Majority Leader Joel Rivera.

Rivera has not officially confirmed if he will run in the special election, but the Norwood News spoke to Rivera last weekend at an event. When asked if he was running, Rivera simply smiled and said, “We will make a formal announcement on Monday [March 2].”

On Monday (and Tuesday), however, calls to Rivera’s office were not returned. Rivera’s wife, Valerie Vazquez, the director of communications and public affairs for the Board of Elections, was unable to answer general questions about the special election. “I recuse myself because my husband is Joel Rivera,” Vazquez said.

Politics runs in the family for Diaz and Rivera. Their fathers, State Senator Ruben Diaz, Sr. and Assemblyman Jose Rivera, the former chair of the Bronx Democratic Party, have strong political ties in the Bronx.

Council Member Larry Seabrook is exploring a run, though time is running out. “Yes, he is interested,” said Allison Crouch, media relations director for Seabrook. “It looks as if he will [run].” Crouch confirmed that Seabrook has established an exploratory committee.

Despite earlier rumors, Council Members Jimmy Vacca and Maria del Carmen Arroyo have both confirmed that they won’t run in the special election or the regular election in the fall.

Both Vacca and Arroyo are not publicly supporting a candidate — yet. “I haven’t decided,” Arroyo said. “There is a lot to discuss.”

Calls to Council Member Helen Foster were not returned, though the Daily News has reported that Foster will not run in the special election and that she is supporting Diaz.

Several sources have also reported that Republican District Leader Anthony Ribustello, best known for his role in the Sopranos, will also enter the race.

The new borough president will finish Carrión’s term and serve until Dec. 31.