A small but growing group of Republican lawmakers are calling on President Trump to begin the transition of power to President-elect Joe Biden, even as the incumbent refuses to concede and forges ahead with a series of legal challenges trying to overturn the results of the Nov. 3 election.  

Sen. Lisa Murkowski, R-Alaska, became the latest GOP senator late Sunday night to issue a statement pushing for the Trump administration to start the presidential transition process.

"President Trump has had the opportunity to litigate his claims, and the courts have thus far found them without merit," she said. "A pressure campaign on state legislators to influence the electoral outcome is not only unprecedented but inconsistent with our democratic process. It is time to begin the full and formal transition process." 

Her comments came shortly after several of her Senate colleagues said it was time to start the transition process while acknowledging Biden as the likely victor of the presidential election. 

Sen. Kevin Cramer, R-N.D., said Sunday on NBC's "Meet the Press" that it's "past time to start a transition, to at least cooperate with a transition. I'd rather have a president who has more than one day to prepare."

Sen. John Cornyn, R-Texas, tweeted Sunday that Biden should start receiving intelligence briefings. Sen. Pat Toomey, R-Penn., said Trump has "exhausted all plausible legal options" in Pennsylvania and urged him to concede. 

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"To ensure that he is remembered for these outstanding accomplishments, and to help unify our country, President Trump should accept the outcome of the election and facilitate the presidential transition process," Toomey said in a statement after a "longtime conservative Republican" judge, Matthew Brann, threw out the latest Trump campaign lawsuit that challenged the Pennsylvania vote.

Trump berated the Pennsylvania Republican on Twitter, saying that “Senator Pat ‘No Tariffs’ Toomey” was “no friend of mine.”

Toomey is retiring rather than running for re-election in 2022.

Rep. Liz Cheney, chair of the House Republican Conference, also said it was time for Trump to respect the "sanctity of our electoral process" if he cannot provide "genuine evidence" of criminality and widespread fraud in the voting process. 

"If the President cannot prove these claims or demonstrate that they would change the election result, he should fulfill his oath to preserve, protect and defend the Constitution of the United States by respecting the sanctity of our electoral process," the Wyoming Republican said in a statement to Politico

Rep. Fred Upton, R-Mich., said during an interview on CNN that "the voters have spoken" and there has been no evidence of fraud or abuse in the election. 

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"All 83 counties have certified their own election results," Upton said of Michigan, a key battleground state where Trump is trying to invalidate voting results. "Those will be officially tabulated or should be tomorrow. We expect that that process move forward and let the voters, not the politicians speak.” 

Trump's efforts to overturn the election results also sparked criticism from his longtime ally and former New Jersey Gov. Chris Christie, who called the president's legal team "outrageous" and a "national embarrassment." 

"I’ve been a supporter of the president's. I voted for him twice, but elections have consequences, and we cannot continue to act as if something happened here that didn't happen," he said on ABC. 

Other Republicans who have publicly congratulated Biden and Vice President-elect Kamala Harris, or who have called on the transition process to begin, include Sens. Mitt Romney of Utah, Ben Sasse of Nebraska and Susan Collins of Maine. 

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Romney issued one of the most scathing statements, accusing him of trying to "subvert the will of the people and overturn the election." 

"It is difficult to imagine a worse, more undemocratic action by a sitting American President," Romney said last week.