Kushner is being interviewed by the Senate Intelligence Committee on Monday.
Grassley and Feinstein said in their statement: "Glenn Simpson, through his attorney, has declined to voluntarily attend Wednesday's Judiciary Committee hearing regarding compliance with the Foreign Agents Registration Act".
Trump tweeted Saturday: "While all agree the U. S. President has the complete power to pardon, why think of that when only crime so far is LEAKS against us".
Democrats did not escape Trump's twitter rantings as he turned his attention for a moment to the ongoing investigation into Russian Federation meddling in the presidential election.
Trump has long criticized leaks about the investigation and has urged authorities to prosecute leakers.
Senate Panel Chairman Chuck Grassley had previously requested the two testify publicly before the committee regarding a 2016 meeting with a Kremlin-linked lawyer who promised damaging information on Hilary Clinton made by Trump Jr during the 2016 presidential campaign.More news: Lewandowski: Trump Jr. Meeting With Russians 'Wasn't a Big Deal'
Trump Jr also recently hired Alan Futerfas, a criminal defense attorney with 25 years experience, who has frequently represented clients in cybersecurity cases.
Collins has challenged Trump and Senate Republicans a number of times.
Congressional lawmakers say President Donald Trump's eldest son and his former campaign chairman won't testify publicly next week as part of the Russian Federation election meddling investigation.
Judiciary Committee Chairman Chuck Grassley and top Democrat Dianne Feinstein warned in letters that the men need to "preserve all relevant documents" in their possession "related to Russian interference in the 2016 election, including documents related to your or the Trump campaign's contacts with Russian government officials, associates or representatives".
After railing against his Democrat opponents and the media, Donald Trump has changed tack and turned his fire on Republican colleagues with a Twitter message complaining about a lack of support from his own party.