Assange fled to the Ecuadorian embassy in London in 2012, seeking asylum from possible extradition to the USA, where he faced indictment under the Espionage Act for publishing leaked government documents.
The self-styled radical transparency group, which USA intelligence believes is a catspaw of Russian intelligence, quoted its legal team to say it is "considering the offer but testimony must conform to a high ethical standard," presumably meaning on the part of the Senate staff who would conduct the interview.
"As part of that inquiry, the committee staff requests that you make yourself available for a closed interview with bipartisan committee staff at a mutually agreeable time and location".
Moreno told the Spanish newspaper El Pais in an interview in the last week of July that Ecuador was not helping Assange to exercise his rights by prolonging his stay in Ecuador's London embassy, and therefore had to find a way to end the impasse.
He was granted asylum in 2012 to avoid extradition to Sweden, where he was accused of sex crimes, allegations that were later dropped.
Assange believes that would pave the way for extradition to the United States for the publication of a huge cache of USA diplomatic and military secrets on the WikiLeaks website.
In the run-up to the 2016 U.S. presidential election, WikiLeaks published documents hacked from Democratic candidate Hillary Clinton's campaign.
As for the question of whether he would be granted full asylum in Ecuador, Valencia said that this is a complex issue that requires unlimited time, because his decisions necessary to reach an agreement between the government of Britain, Ecuador and lawyers of the person involved.
His testimony could be useful to special counsel Robert Mueller, who is leading the "bipartisan" investigation into Russian meddling Trump has branded a "witch hunt".