U.S. President Donald Trump on Monday rejected the Democratic-led House of Representatives' impeachment charges and called for their immediate dismissal by the Republican-led Senate in a memo offering a legal and political case against his removal. 

The 116-page Trial Memorandum sought to undercut charges that the Republican president abused his power and obstructed Congress, and constituted Trump's first comprehensive defense before his Senate trial begins in earnest on Tuesday. 

"The Senate should reject the Articles of Impeachment and acquit the president immediately," the memo concluded.

Trump has been impeached on two charges. The first is abuse of power over efforts to pressure the president of Ukraine to launch investigations into Trump's political rival while withholding nearly 400 billion U.S. dollars in military aid. 

The second charge is obstruction of Congress for withholding testimony and documents from investigators. 

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Democrats said Trump abused his power by withholding U.S. military assistance to Ukraine as part of a pressure campaign and obstructed Congress by refusing to hand over documents and barring administration officials from testifying, even when subpoenaed by House investigators. 

Democratic Senate Minority Leader Chuck Schumer said, "The charges are serious, let's make no mistake about it. If a president can interfere and get... if a president can get a foreign power to interfere in our elections and determine the outcome that is against the whole essence of what America is all about. The American people should determine who their president and senator and congressmen are, not a foreign power."

While Trump's defense argued neither charge constituted a crime or impeachable offence, that he was within his rights as president to make decisions about foreign policy and what information to give Congress, and that the House pursued a flawed and one-sided process before impeaching him on December 18. 

"House Democrats settled on two flimsy Articles of Impeachment that allege no crime or violation of law whatsoever – much less 'high Crimes and Misdemeanors,' as required by the Constitution," it said. "They do not remotely approach the constitutional threshold for removing a president from office."

The memo's executive summary asserted that the House Democrats' "novel theory of 'abuse of power'" was not an impeachable offence and supplanted the Constitutional standard of "Treason, Bribery, or other high Crimes and Misdemeanors." 

It rejected the obstruction of Congress charge as "frivolous and dangerous," saying the president exercised his legal rights by resisting congressional demands for information, also known as subpoenas. 

It also accused the House Democrats of conducting a rigged process, said they succeeded in proving only that Trump had done nothing wrong and argued, as the White House has repeatedly, that this was an effort to overturn Trump's 2016 election victory and to prevent his re-election in November. 

While the Republican-controlled Senate is highly unlikely to remove Trump from office, it is important for him to try to diminish the Democratic accusations to limit political damage to his bid for a second term. 

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In part of his legal filing, Trump's lawyers cast their arguments in unabashedly political terms. 

"They want to use impeachment to interfere in the 2020 election. It is no accident that the Senate is being asked to consider a presidential impeachment during an election year," the memo said. 

"Put simply, Democrats have no response to the president's record of achievement in restoring prosperity to the American economy, rebuilding America's military, and confronting America's adversaries abroad," it added. 

The president also has repeatedly dismissed the impeachment as a political witch hunt. 

"We're achieving what no administration has ever achieved before, and what do I get out of it? Tell me. I get impeached. That's what I get out of it – by these radical left lunatics – I get impeached," Trump said. 

One big question is whether the Senate will hear from witnesses. Democrats want administration officials like former U.S. National Security Adviser John Bolton to testify. 

Some Republicans appear to support having witnesses testify, but they will also likely push to hear from witnesses such as former vice president, and current presidential candidate, Joe Biden and his son Hunter, who had business dealings in Ukraine. 

(With input from agencies)