Mitch McConnell, the Republican leader in the Senate, promised on Thursday that the transition after the US presidential election will be orderly.
President Donald Trump said a day earlier that he wanted to wait and see how the elections go.
"The winner of the November 3 election will be inaugurated on January 20," McConnell wrote on Twitter.
His tweet was prompted by statements by Trump, who did not want to say on Wednesday whether he will accept the election results.
"We will see what happens," said the incumbent president.
This year, more Americans than ever will vote by mail.
Trump doubts this method is safe enough.
The discussion about postal voting led to a showdown between Democrats and Republicans in August.
Trump: 'Postal voting is detrimental to Republicans'
The US president says postal voting will be detrimental to him and the Republicans.
However, experts argue that studies have not shown that this method is more beneficial to either party.
A White House spokesman said on Thursday that the president will accept the results of a "free and fair election."
But since the US president has long said the elections will be unfair, this commitment does not reassure politicians in Washington.
Many of McConnell's party members support his message and say they will accept the election results.
"I can guarantee you the transition will be peaceful," Senator Lindsey Graham told
Democrats also emphasize that they expect a fair transition in the event of a loss of Trump.
See also: Why are the Democrats and Republicans arguing about the postal service?