Vermont Sen. Bernie Sanders has been a vocal critic of President Donald Trump through the latter’s campaign phase and presidency, and now the country's most popular senator has said he believes Trump will not be re-elected in 2020.

“In terms of the first three months in office, Donald Trump is the least popular president in the history of polling,” Sanders told the Associated Press on Wednesday. “I do not believe that if Trump continues these policies that he's going to be re-elected. Nor do I think that the Republicans are going to do well in 2018.”

Read: Bernie Sanders Slams President Donald Trump's Airstrike On Syria

Sanders, who was a Democratic presidential nominee in the 2016 election, announced his bid for re-election in Vermont on Tuesday in a letter that asked supporters to help him raise funds for a nationwide tour to rally against the Trump administration’s policies next week.

The letter, an unusual way to announce that he will be running again, read, “Please make a $27 contribution to my re-election campaign to help fund this national tour.”

The tour, during which Democratic National Committee Chairman Tom Perez will be joining Sanders, will kick off Monday with a rally in Portland, Maine. Over the week, the liberal icon has appearances scheduled for Kentucky, Florida, Texas, Nebraska, Utah, Nevada and Arizona.

“The momentum right now is with the progressive movement in this country,” Sanders told the agency. “And I think the Republicans are on the defensive and will be on the defensive increasingly.”

Sanders gave the example of the Kansas’ special election Tuesday, where Republican candidate Ron Estes pulled a narrow victory against civil rights attorney James Thompson, a Democrat, for the House seat vacated by CIA Director Mike Pompeo. Despite the loss, it was a major feat for the Democrats to put up a strong fight in a region that was won by President Trump by an overwhelming margin last year.

“The Republicans had to spend money like crazy at the end to beat him,” Sanders said. “I think that's a very good omen for the future.”