At the weekend a milestone was reached in the investigation into the United States 2016 election. US prosecutors for the first time directly tied President Donald Trump to a crime committed by a lieutenant in a case involving hush money payments to two women who had alleged affairs with Trump.

During legal filings on campaign finance violations committed by former personal lawyer Michael Cohen, the documents say they were "in coordination with and at the direction of" Trump, referred to as "Individual-1". Trump has not been charged with a crime. Candidates are required to report any payments made to influence the election under campaign finance law. Trump's campaign didn't do so.

It's arguable whether public knowledge of the alleged affairs and cover-up would have doomed Trump. The Access Hollywood tape and claims of wrongdoing from other women did not derail Trump's campaign. Federal prosecutors in the Southern District of New York are investigating the campaign violations, alongside the wider Russia probe. Overall, the Russia probe shows that various officials in the Trump campaign responded enthusiastically behind the scenes to contacts from Russia, going back to November 2015.

What does this mean going forward? The US Justice Department has a policy that a sitting president cannot be indicted. But the alternative question of impeachment is also complicated.

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The new Democratic-controlled House of Representatives from January will be able to pursue impeachment. But it's unlikely the party would be able to secure enough Republican support in the Senate for removal. Would it be smarter politically, instead, to hold hearings into Trump's dealings without going all out on impeachment?

The Democrats achieved a Midterm House win by focusing on issues that matter to voters rather than Trump. It's unlikely that expected speaker Nancy Pelosi will lose sight of that.

A new NPR/PBS/Marist poll shows the top priorities for the new Congress among voters.

For Republicans it is immigration (35 per cent), economy and jobs (17 per cent), healthcare (11 per cent), and foreign policy and terrorism (10 per cent). For Democrats it is healthcare (24 per cent), economy and jobs (16 per cent), climate change (15 per cent), and guns (15 per cent).

Republicans in Congress will be wondering what the Russia probe and a chaotic White House throw up next. Yesterday it was the news that White House Chief of Staff John Kelly is leaving, after months of trying to contain his headstrong, impulsive boss.

The drums for the 2020 Democratic presidential primary are already beating. The Democrats will have a better chance of making Trump a one-term president if they keep their heads down and govern. Missteps could play into Republican hands.

The party heads towards 2020 with uncertainty over the economy and a Midterm report card that highlighted a Republican revolt in suburban areas.

But Trump has seen off previous opponents who get drawn into one-on-one combat, giving him confidence against whoever is the Democratic nominee.