The Democratic U.S. lawmakers leading the impeachment case against Republican President Donald Trump said on Saturday the president must be removed from office to protect national security and preserve the country's system of government.

The Democratic House of Representatives impeachment managers faced a deadline of 5 p.m. EST (2200 GMT) on Saturday to file the document, before the trial of the Republican president starts in earnest in the Senate on Tuesday.

In a brief filed ahead of a Saturday deadline, the lawmakers laid out their arguments supporting charges of abuse of power and obstruction of Congress against the president in a 111-page document.

"The Senate should convict and remove President Trump to avoid serious and long-term damage to our democratic values and the nation's security," the lawmakers said, for the first time formally calling for the Senate to convict the president and remove him from office.

"The case against the president of the United States is simple, the facts are indisputable, and the evidence is overwhelming," the lawmakers said.

They also called for Trump's removal in order to safeguard the integrity of the 2020 U.S. general election, in which Trump is seeking re-election, as well as the U.S. system of government.

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Trump's legal team issued a resounding rejection of the House of Representatives' impeachment, calling the charges a "dangerous attack on the right of the American people to freely choose their president."

"This is a brazen and unlawful attempt to overturn the results of the 2016 election and interfere with the 2020 election – now just months away," they argued in a six-page document released on Saturday.

"The case against the president of the United States is simple, the facts are indisputable, and the evidence is overwhelming," they said.

As well as the charge of abuse of office for pressuring Ukraine to investigate Biden and his son Hunter Biden, Trump is also accused of obstructing Congress in its investigation into his conduct.

"An acquittal would also provide license to President Trump and his successors to use taxpayer dollars for personal political ends," the Democratic lawmakers' brief said.

Trump has denied wrongdoing and has accused Democrats of a partisan-driven effort to undo his 2016 election victory.

The Senate trial is unlikely to lead to Trump's ouster, as no Republican senators have voiced support for doing so.

The Trump lawyers, in their document, argued that Trump acted at all times with full constitutional legal authority, said one of three sources close to Trump's legal team who briefed reporters on a conference call on Saturday.

"We will take the facts head on and we believe that the facts will prove, and have proven, that the president did absolutely nothing wrong," the source said.

Trump's trial brief is due at noon (1700 GMT) on Monday, and the House's reply brief is due on Tuesday at noon (1700 GMT).

(With input from Reuters)