'Squad members' Alexandria Ocasio-Cortez and Ilhan Omar endorse Bernie Sanders in 2020 race

Leading members of the "squad", the influential group of liberal congresswomen, are endorsing Bernie Sanders in a boost for the 78-year-old as he returns to the campaign trail following a heart attack.

Congresswoman Ilhan Omar called Mr Sanders the "best candidate to take on Donald Trump" as she announced she was backing him for the Democratic nomination in the 2020 election on Tuesday night.

Her fellow "squad" members Alexandria Ocasio and Rashia Tlaib are also thought to be preparing to endorse the Vermont senator.

In her announcement, Ms Omar said Mr Sanders was the "only candidate" to have built a  "movement... that transcends generation, ethnicity, and geography".

The endorsement was timed to coincide with the end of the latest Democratic debate in Ohio on Tuesday night, in which Mr Sanders faced questions about his age and health.

During the debate Tuesday night in Ohio, Mr Sanders sought to assure voters that he is fit enough to run for president despite having suffered a mild heart attack recently.

And he said he would be joined by a "special guest" at a big campaign rally on Saturday in the New York borough of Queens.

That guest is thought to be Ms Ocasio-Cortez, the most high-profile member of the "squad" and the face of the progressive wing of the Democratic Party. The New York electoral district she represents includes part of Queens.

Ms Ocasio-Cortez, as she is popularly known, calls herself a Democratic socialist like Mr Sanders, who is nearly 50 years her senior and has held a seat in Congress since 1991.

Ms Tlaib suggested on Twitter that she too is preparing to endorse Mr Sanders. The endorsement plans of the fourth member of the Squad, Ayanna Pressley, are not immediately known.

The "Squad", an outspoken, leftist group has been a frequent target for Mr Trump. They gained national recognition after Mr Trump famously told them to go back to the "crime infested places from which they came." Except for Ms Omar, who was born in Somalia and came to the US as a refugee, all of the members were born in America.

Mr Sanders had been polling second-place in the Democratic race behind former vice president Joe Biden, seen as a moderate, but has recently been overtaken by fellow progressive senator Elizabeth Warren.

The Sanders campaign is hoping for a boost from the endorsement of the young, racially diverse and influential congresswomen as he battles to catch up in the polls with Mrs Warren.

In 2016 he struggled to gain traction with African Americans and other ethnic minorities but his campaign has focused on trying to woo younger and more racially diverse groups going into 2020.

Mr Sanders has already won the endorsement of the rap artist Cardi B, who is from the Bronx, another New York borough that includes part of Ms Ocasio-Cortez's electoral district.