Democrats and Republicans closed ranks on Tuesday a day ahead of the expected impeachment of US President Donald Trump, underscoring the country's deep political divide over charges that the US leader abused his power.Trump sang his own praises on Twitter, citing the economy's strength and trade negotiation successes even as he faced becoming only the third president ever with the impeachment stain on his record.While opinion polls showed the US electorate closely divided over impeachment, Trump flagged a USA Today survey saying he remained popular and could defeat any of the leading Democrats seeking to challenge him in the November 2020 elections."The new USA Today Poll, just out, has me leading all of the Democrat contenders... The American people are smart. They see the great economy, & everything else!" he tweeted.Critics of the poll pointed to its unusual methodology - pitting Trump against a named Democrat and an imaginary, unnamed "third-party candidate" with no record and no political positions in each question.Impeachment 'a joke' Meanwhile Democrats held tight to the path that could see Trump go on trial in the Senate for abuse of power and obstruction of Congress in January.A handful of moderate House Democrats who risk electoral defeat next November for their anti-Trump stances declared their support for impeachment, virtually ensuring Trump will be forced to stand trial."My military service taught me to put our country - not politics - first, and my time as a federal prosecutor taught me about the importance of the rule of law and of justice," declared Mikie Sherrill, a first-term representative from a Trump-leaning district in New Jersey."I will be voting in favour of the articles of impeachment.""I know some people will be angry at my decision, but I was elected to do what is right, not politically safe," said Anthony Brindisi, another first-term Democrat, from a conservative new York constituency.Their pledges ensured Democrats, who hold 233 seats in the House of Representatives, will be able to vote through the historic charges against Trump over a unified Republican Party, which has 197 seats in the body.Currently just two Democrats are expected to vote against impeachment, one of whom is also believed to be mulling switching parties entirely. The House Rules Committee was expected to finalise the impeachment articles on Tuesday, and then a debate and vote are expected to be scheduled for the House floor on Wednesday.Republicans stuck to their arguments that the charges that Trump illicitly solicited help for his 2020 reelection effort from Ukraine were groundless and did not meet the constitutional grounds for impeachment.Even before the trial, Republican senators -- who are supposed to sit as impartial judges in the trial -- began declaring their positions."The House impeachment articles are a joke," Senator Josh Hawley told Fox News. "This whole thing is a joke and it's time to get the president exonerated."'Fishing expedition' The leaders in the Senate meanwhile began battling over the process of the trial, and especially whether the chamber would call as witnesses four current or former Trump aides whom the White House blocked from testifying and providing documents to the House investigation.On Sunday Chuck Schumer, the top Democrat, urged Republican Senate Majority Leader Mitch McConnell to allow the testimony of White House Acting Chief of Staff Mick Mulvaney, Mulvaney advisor Robert Blair, former national security advisor John Bolton and budget office official Michael Duffy.Schumer says the four have "direct knowledge" of the thinking behind Trump withholding military aid to Ukraine, allegedly to pressure Kiev to announce investigations into the president's Democratic rivals.McConnell dismissed Schumer's request as premature and politically driven, calling it "a fishing expedition" that underscored Democrats' search for evidence to support impeachment.He called the house case for charging and removing Trump from office "the thinnest, least thorough presidential impeachment in our nation's history"."It's not the Senate's job to leap into the breach and search desperately for ways to get to guilty. That would hardly be impartial justice," he said.