Dillon was well known for his opposition to abortion rights, and the issue prompted his defection to the Republican Party in 1989, having previously been one of the very few Democratic politicians to have success in Nassau County.

Dillon was born in 1933 into a devout Roman Catholic family in the Bronx, and spent parts of his childhood living in the borough's Woodlawn Heights neighborhood, where his father owned a bar, as well as the Rockaway Beach section of Queens and in Arlington, Virginia, an immediate suburb of the District of Columbia.

[2][3][4] Dillon was originally elected in 1974 as a Democrat, defeating 12-year incumbent Republican William Cahn and Conservative Party nominee Francis B. Hearn with 52% of the vote in what The New York Times called a "major upset".

The campaign between Dillon and Cahn was brutal from the onset, and took place amidst the backdrop of the January 10, 1974, revelation that Cahn was under investigation by the office of the United States Attorney for the Eastern District of New York, where Dillon served as Organized Crime Task Force chief, for alleged grand jury tampering in relation to the revocation of indictments against three Republican Oyster Bay municipal officials involved with parking meter kickbacks.

The investigation was opened in August 1973, when Dillon was approached with the allegations by Norman E. Blankman, an independent candidate for County Executive on the Integrity line, but the matter was quickly reassigned to another assistant by US Attorney Robert A. Morse because it lacked a nexus to organized crime.

Morse's death left the US Attorney's office vacant, pending the recommendation of a new appointee to President Richard Nixon from New York's two Republican senators, Jacob Javits and James L. Buckley.

[6] When word of Margiotta's recommendation that Margolin fill the vacancy reached assistants in the US Attorney's office, they contacted Department of Justice officials in Washington to express their concern that he could interfere with the investigation of Cahn.

Margolin was soon dropped from consideration upon the disclosure that he had been subpoenaed to appear before the federal grand jury investigating Cahn, with the appointment instead going in late March to David G. Trager, a professor at Brooklyn Law School.

[8] Dillon's candidacy was endorsed by the county Democratic committee on the first ballot in a June meeting, despite concerns from some that his status as a recent transplant to Rockville Centre from New York City made him a carpetbagger.

Dillon ultimately declined the opportunity, instead accepting an offer brokered by his friend, then-Hempstead Town Councilman Peter King, of the Republican endorsement and ballot line for re-election as District Attorney.

In the aftermath of Dillon's decision, Democrats largely abandoned their countywide effort for the November election, running their county vice chairman John Matthews as a sacrificial lamb against Purcell.

This sequence of events led to the Democrats planned nominee for Comptroller, Richard Kessel, dropping out of the race, saying that the electorate would "vote for Purcell on the Republican line, Dillon on the G.O.P.

"[24][25] Baird's challenge, as well as an appeal from the Nassau chapter of the National Organization for Women for voters to write-in Jane Roe, combined to just 1% of the total votes cast, but the size of Dillon's undervote also increased by 67,000 from 1981, when he also ran unopposed with all five ballot lines.