Montana Gov. Steve Bullock Steve BullockSenate Democrats demand White House fire controversial head of public lands agency Pence seeks to boost Daines in critical Montana Senate race Trump's fear and loathing of voting by mail in the age of COVID MORE (D) is likely to challenge Sen. Steve Daines Steven (Steve) David DainesSenate Democrats demand White House fire controversial head of public lands agency Pence seeks to boost Daines in critical Montana Senate race The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support MORE (R-Mont.) this year, reversing his position after a ferocious lobbying campaign from top Democrats who hope he can put another Senate seat in play.

Bullock’s decision, first reported by The New York Times, comes just days before the state’s filing deadline. He had resisted the idea of running for Senate for months; Bullock told The Hill as recently as February that he would serve out the end of his term and that he would not run.

But a source close to Bullock said he’s likely to announce a Senate run on the filing deadline on Monday after the governor talked with his family.

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During a recent visit to Washington, Bullock sat down with former President Obama, who urged the Montana governor to run for the Senate seat, according to a source familiar with the meeting. Also last month, Senate Minority Leader Charles Schumer Chuck SchumerDemocrats scramble on COVID-19 relief amid division, Trump surprise Pelosi, Schumer 'encouraged' by Trump call for bigger coronavirus relief package Schumer, Sanders call for Senate panel to address election security MORE (D-N.Y.) flew to Montana to make a last-minute appeal.

The lobbying campaign, and assurances from top party leaders that he would have the financial and political support necessary to mount a strong campaign, helped change Bullock’s mind.

Still, Bullock faces an uphill climb ahead. President Trump Donald John TrumpBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Military leaders asked about using heat ray on protesters outside White House: report Powell warns failure to reach COVID-19 deal could 'scar and damage' economy MORE is all but certain to win Montana’s electoral votes in November; he beat Hillary Clinton Hillary Diane Rodham ClintonBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Virginia Democrat blasts Trump's 'appalling' remark about COVID-19 deaths in 'blue states' The Hill's Campaign Report: Biden asks if public can trust vaccine from Trump ahead of Election Day | Oklahoma health officials raised red flags before Trump rally MORE by 20 points in Montana in 2016. Daines won his seat with nearly 58 percent of the vote in 2014, and he ended the year with more than $5 million in the bank.

But Bullock’s entry will make Daines’s life more difficult. Five little-known Democrats are challenging the incumbent, none of whom have raised more than half a million dollars.

Bullock has a new political donor list after his brief run for the Democratic presidential nomination. A recent survey conducted by the University of Montana showed Daines winning 47 percent of the vote and the Democratic candidates, all tested in the same question, pulling a combined 53 percent of the vote.

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Republicans have been preparing for a potential Bullock candidacy, testing messages against him in private surveys. President Trump has tweeted about Daines several times in recent days, a sign, top Republicans said, that Trump would use his coattails to pull Daines along.

“The great people of Montana can have no better VOICE than Senator @SteveDaines. He is doing an incredible job! Whoever the Democrat nominee may be, please understand that I will be working hard with Steve all the way,” Trump tweeted Wednesday.

And Republicans signaled they would accuse Bullock of failing to warn a future employer of sexual harassment allegations against a former top political aide. That aide, fired from the Democratic Governors Association in 2015, later resigned from New York City Mayor Bill de Blasio Bill de BlasioDe Blasio to furlough himself, 494 other staff members amid financial crunch: report The Hill's 12:30 Report - Presented by Facebook - Trump's West Coast campaign swing NY cancels traditional Macy's Thanksgiving parade, will hold virtual event MORE’s office in the face of new harassment allegations.

Alex Bolton contributed reporting.