Maine's Democratic US Senate nominee Troy Jackson is refusing to apologize for telling incumbent Republican Senator Susan Collins to 'shut up' during a fiery debate last week.

Jackson doubled down on the explosive exchange during an interview with CNN's 'State of the Union' on Sunday, insisting he had no regrets about his conduct after Collins accused him of being connected to a protester who disrupted their debate.

'I mean, she accused me of setting that up. I had no idea who it was,' Jackson told host Dana Bash.

'I've had four debates with Senator Collins. I've shook her hand afterward every time. But what I'm not going to be is talked down to by Senator Collins. I'm not going to, you know, be accused of things like that.'

The clash erupted Thursday night when a protester opposed to data centers interrupted Collins as she attempted to answer a question.

As moderators moved to have the protester removed, Collins took a swipe at her Democratic rival.

'So this is obviously one of Troy's supporters,' she said.

'Oh shut up,' Jackson fired back. 'I don't even know that kid,' to which Collins responded, 'Oh, right.'

The protester, 19-year-old Bowdoin College student Henry Elgie, later told the New York Times that he does not personally know Jackson, 'but I think he should be winning.'

Jackson's outburst quickly sparked fury among Republicans, with the Maine GOP branding his behavior 'unacceptable.'

Collins then escalated the feud Friday, suggesting Jackson's remark reflected a broader pattern in how he deals with women who challenge him.

'I was struck by how reflexive the response, 'Oh, shut up,' was,' Collins told Fox News.

'But then I thought about the reputation Troy Jackson has in Augusta, particularly when he's dealing with women with whom he disagrees. So in some ways, it didn't surprise me.'

 The ugly confrontation came at a crucial moment in one of the country's most closely watched Senate battles.

Collins is fighting for a sixth term in Washington while Jackson is attempting to topple the longtime Republican incumbent in a contest that could help decide control of the Senate. 

A fiercely independent Republican who often bucks President Donald Trump's policies and statements, Collins is in the midst of a tough re-election battle in Maine, which has one of the oldest electorates in the country. 

After her previous opponent Graham Platner dropped out of the race amid a slew of allegations of sexual misconduct and abuse, which he denies, Maine Democrats selected Jackson, the former state Senate President, as their nominee. 

However, Jackson has also been marred by his own controversies, including a revelation that he fathered children with his second cousin.

Recent polls have shown Jackson clinging to a razor-thin advantage, while the Cook Political Report rates Maine as one of just seven Senate races considered a 'toss-up.'