US Senator Mitt Romney and other top Republicans have dismissed President Donald Trump's refusal to commit to a peaceful transfer of power.
They also assured American voters the politicians would accept the outcome of November's election.
Mr Trump declined on Wednesday to embrace a peaceful transfer in response to a reporter's question and said he expected his election battle with Democrat Joe Biden to be settled by the Supreme Court.
Speaking to reporters, Mr Trump said he did not know that an "honest" election could be held on 3 November "with this whole situation, unsolicited ballots".
Senator Mitt Romney later said "any suggestion that a president might not respect this Constitutional guarantee is both unthinkable and unacceptable".
By midday, with a controversy raging, White House Press Secretary Kayleigh McEnany told a news briefing: "The president will accept the results of a free and fair election."
But for months, Mr Trump has cast November's election as being rigged and repeatedly attacked Democrats for promoting widespread use of mail-in ballots for voters during the pandemic.
In an interview on Fox News Radio, Mr Trump called mail-in ballots "a horror show", despite studies showing no significant problems with that method of voting over the years.
Michael Waldman, president of New York University's Brennan Center for Justice, said voting arrangements were steadily advancing.
In an interview with Reuters Television, he added:"The system is not broken. States are actually improving their voting rules day by day."
Democrats accused Mr Trump of threatening American democracy and further politicising his upcoming choice to replace the late Supreme Court Justice Ruth Bader Ginsburg by suggesting the yet-to-be named nominee would have a role in the election's outcome.
Some of Mr Romney's fellow Republicans joined the effort to quell election fears, including Senators Marco Rubio and Mitch McConnell and House Republican leader Kevin McCarthy, who told reporters: "It will be a smooth transition regardless of the outcome."
Mr Trump, who trails Mr Biden in national opinion polls, has long sought to cast doubt on the legitimacy of the election, asserting without evidence that mail-in voting would be rife with fraud.
"President Trump, you are not a dictator and America will not permit you to be one," said Senate Democratic leader Chuck Schumer, calling him "the gravest threat" to US democracy.
Senator Bernie Sanders, who lost to Mr Biden in the Democratic Party's presidential nominating race, called for an independent commission to oversee the upcoming election.
Democratic House of Representative Speaker Nancy Pelosi cautioned against panicking over the remarks of a president who she said admires autocratic leaders.
At a news conference, she urged Americans to cast their ballots and admonished Trump: "You are not in North Korea, you are not in Turkey, you are not in Russia."
If November's election is close, Mr Trump could contest the results in federal courts in hopes of being awarded enough Electoral College votes to retain the White House, according to political analysts.
Only one US presidential election, the 2000 contest between Republican George W Bush and Democrat Al Gore, has had its outcome determined by the Supreme Court.
Senate Judiciary Committee Chairman Lindsey Graham, a Trump ally who will play a significant role in whether Mr Trump's upcoming Supreme Court nominee will be confirmed, said there could be litigation over the presidential election.
"The (Supreme) Court will decide and if the Republicans lose, we will accept that result," Mr Graham told Fox News. "But we need a full court."
If Mr Trump nominates a conservative to serve on the Supreme Court, as expected, and the Republican-controlled Senate confirms the nominee, it will have six justices considered to be conservative and three viewed as liberals.