He won a 1948 special election to the United States House of Representatives from New York's twenty-fourth district (Bronx) as the candidate of the American Labor Party in what The New York Times called "a test of Truman-[versus]-Wallace strength" with regard to the upcoming U.S. presidential elections and a "test today of the third-party movement headed by Henry A.

[1][4][6] In 1936, Isacson became a member at the founding of the American Labor Party (ALP, which sought to advance the cause of trade unions).

[6] The Democratic nominee was Karl Propper, a former president of the Bronx Bar Association, and his supporters included Eleanor Roosevelt and New York City Mayor William O'Dwyer.

In contrast, the Amalgamated Clothing Workers of America (ACWA), a more conservative CIO union opposed to Wallace, confirmed that FBI agents had visited many of its locals to investigate possible violations of the Hatch Act of 1939, regarding political activities, and declared the FBI had given the ACWA a "clean bill of health".

Nathan Witt, the council's legal counselor, disagreed, stating, "There could not possibly be any technical violation in 1948 except for the Isacson election and the FBI agents made clear they were not investigating that.

Meanwhile, Adolf A. Berle, co-founder of the recently formed Liberal Party, denounced Wallace as a "front for an international intrigue".

[8]The Washington Post declared "Henry A. Wallace yesterday jolted Democratic hopes of holding New York in November" with the election of the ALP's Isacson and noted that it was "Wallace's first test at the polls since he broke with the Democratic Administration to form a third party and make his own bid for the Presidency".

[16] The New York Times stated that the election of the ALP's Isacson over the Democratic candidate Propper would have many effects, including that: The next day, Wallace announced that his Progressive Party had won a place in the California primaries.

Two days later, The Washington Post was still assessing the implications of Bronx congressional election: The outcome of Tuesday's special election in New York's Twenty-fourth Congressional District confirms what most political commentators have for some time surmised that Henry Wallace, if he wishes to do so, can deprive the Democratic candidate of New York State's 47 electoral votes next November.

[19] The Post also believed that (in retrospect, just four months before declaration of the State of Israel) "According to all seasoned political observers, it was the Palestine issue that gave the victory to Leo Isacson, the Henry Wallace-American Labor Party candidate, in the special congressional election in the Bronx.

[21] On February 23, 1948, The New York Times was still analyzing the election in an article whose headline read "Isacson's Victory Is Aid to Wallace in Major States" with the subtitle "Adds Strength to Third-Party Movement for Presidency, A Survey Discloses".

[26] Isacson joined fellow New York ALP representative Vito Marcantonio and Democrats in voting unsuccessfully against an "unprecedented" $200,000 appropriation to the House Un-American Activities Committee.

[27] In April 1948, he became the first Congressman ever to be denied a United States passport by the State Department when he attempted to go to Paris to attend a conference as an observer for the American Council for a Democratic Greece, a Communist front organization, because of the group's role in opposing the Greek government in the Greek Civil War.

[1] In September 1948, Isacson stumped for Wallace in New York City with fellow ALP candidates Marcantonio (Harlem), Irma Lindheim (Queens), and Lee Pressman (Brooklyn).

"Representative Isacson declared that President Truman would learn during the campaign that he could not hide the responsibility of the Democratic Party for the bi-partisan assault on the liberties of the American people.

"[31] In the fall of 1948, Isacson ran for a full term against Democrat Isidore Dollinger, who also had the Republican and Liberal endorsements.

[4] In the early 1970s, he moved to Tamarac, Florida, and taught as an adjunct professor of political science at Nova Southeastern University.