Bernie Sanders has asked what “the Russians have on" Donald Trump and asked why the US President has “nothing but nice things” to say about Vladimir Putin.
The former Democratic presidential nominee took to Twitter to question his leader's links with Moscow.
The US government alleged last summer that Russian intelligence had tried to interfere with the presidential election, in an apparent attempt to help Mr Trump.
This included hacking into the emails of senior members of Hillary Clinton’s team and the Democratic National Committee.
In late December, Mr Obama expelled 35 Russian diplomats in punishment for the alleged interference. Vladimir Putin surprised many by deciding not to reciprocate.
Mr Trump has long rejected the claim that Russia had come to his assistance or the member of his campaign team colluded with the Kremlin. Moscow also denied the allegations.
Mr Sanders remains unconvinced.
“How does it happen that we have a president who has nothing but nice things to say about Mr Putin? What do the Russians have on Mr Trump," he asked.
The Vermont Senator has previously called for a transparent investigation into former national security adviser, Michael Flynn’s contact with the Russian ambassador, Sergey Kislyak.
Attorney General Jeff Sessions has also come under scrutiny for failing to disclose his two meetings with Mr Kislyak.
