Sen. Susan Collins Susan Margaret CollinsClub for Growth to spend million in ads for Trump Supreme Court nominee Maryland's GOP governor says Republicans shouldn't rush SCOTUS vote before election The Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November MORE (R-Maine) said Sunday that there's "nothing wrong" with a crop of Republicans considering a primary challenge to President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE in 2020.

"I see nothing wrong with challengers," she said on CNN's "State of the Union." "That is part of our democratic system."

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Several prominent Trump critics within his own party are considered possible candidates in 2020, most notably Ohio's GOP Gov. John Kasich. Kasich says he's "very seriously" considering a run.

"It's up to those individuals to decide whether or not they're going to oppose the president," Collins continued. "They would probably have an uphill climb."

Collins also declined to endorse Trump as the Republican nominee, saying she's not ready to talk about 2020 yet.

Collins, who is considered a top target for Democrats in 2020, said "I'm going to talk about 2020 in 2020" and focus on the politics of the day now.

"I'm worried that we're getting into perpetual campaign mode," she said.

Republican Sen. Susan Collins of Maine welcomes 2020 primary challengers to President Donald Trump, saying that it's "healthy" for democracy.



"I see nothing wrong with challengers,” Collins said. https://t.co/TUVWySq4eT pic.twitter.com/htIcK1tnWt — State of the Union (@CNNSotu) December 16, 2018

Several sitting presidents have been challenged in the primary for their second term, most recently former President George H.W. Bush in 1992 by Pat Buchanan.

Collins also discussed Trump's former attorney, Michael Cohen, and federal prosecutors implicating the president in campaign finance violations, saying it was "not a good week" for Trump.

"Let me point out that there are a lot of unanswered ethical, legal and factual questions but clearly this was not a good week for President Trump, nor for his campaign organization and these allegations are concerning," she said. "But we need to wait until we have the entire picture. And that's why it's so critical that the special counsel be allowed to complete his investigation unimpeded so that we can have the full picture."

Cohen was sentenced last week to three years in federal prison for a series of crimes he committed while working for Trump, including campaign finance violations stemming from payments to two women alleging to have had affairs with Trump.

--This report was updated at 12:46 p.m.