* Trump fighting to keep his financial records private
* Supreme Court to hear three pivotal cases in March
* Justices hemmed in previous presidents in major rulings
By Lawrence Hurley and Jan Wolfe
WASHINGTON, Feb 6 (Reuters) - In federal court and on the Senate floor, lawyers for President Donald Trump have argued that the U.S. Constitution confers on a president broad protection from scrutiny by Congress, prosecutors and the judiciary for his actions.
This expansive view of presidential powers may have helped the Republican president win acquittal on Wednesday in his Senate impeachment trial, but a raft of court rulings due in the coming weeks and months could have an even more profound impact on setting the parameters for a president's authority.
Three cases will be argued before the Supreme Court on March 31 focusing on Trump's contention that a House of Representatives committee and a New York City prosecutor are powerless to enforce subpoenas to obtain his financial records. Lawyers for Trump argued that the Constitution renders sitting presidents immune from all criminal investigations and that even if he shot someone on New York's Fifth Avenue prosecutors would be powerless to act while he was still in office.
The Supreme Court eventually could be called upon to decide another major case now awaiting a ruling in the U.S. Court of Appeals for the District of Columbia Circuit. In that one, the Justice Department argued that senior presidential aides are immune from congressional subpoenas for testimony like one issued by the Democratic-led House Judiciary Committee to former White House Counsel Don McGahn.
The rulings in those cases could set enduring legal precedents.
In impeachment trials, "no current Congress can bind a future Congress," said University of Missouri law professor and impeachment expert Frank Bowman, meaning whatever arguments Trump's lawyers advanced applied only to that trial.
"Courts are different. They have got to explain. And, at least in theory, what the high court decides binds all lower courts," Bowman added.
The Supreme Court previously curbed presidential power in key cases.
In 1997, it ruled that President Bill Clinton could not evade a civil lawsuit brought by Paula Jones, a woman who accused him of making unwanted sexual advances. In 1974, it ruled against President Richard Nixon's bid to avoid handing over audiotapes and other material to a federal judge as part of a criminal investigation into the Watergate corruption scandal.
During the impeachment trial, Trump's legal team argued for an unconstrained presidency. They also said that the charges brought by the House - abuse of power and contempt of Congress - did not cover conduct that amounted to impeachable offenses.
They argued that if a president trying to win re-election believes he is acting in the national interest, any quid pro quo - a Latin term meaning a favor for a favor - arrangement aimed at boosting his election chances cannot be impeachable. They also argued that an impeachable offense must be a violation of a statutory law.
Democrats said this sweeping assessment of presidential power would embolden Trump and future presidents to act with impunity.
'EXTREME AND RIDICULOUS'
It remains to be seen whether such arguments will win favor at the Supreme Court, whose 5-4 conservative majority includes two justices appointed by Trump.
"I doubt there's five votes for the more extreme and ridiculous arguments made by the president's lawyers in the Senate," said Ilya Somin, a professor at George Mason University's Antonin Scalia Law School in Virginia.
Some of the Trump team's statements at the impeachment trial, Somin added, "may damage their credibility."
In the McGahn case, Trump administration lawyers argued that Congress cannot force a presidential aide to testify even in its usual oversight role, and that impeachment is the only real power Congress has to police a president's conduct. Trump directed current and former officials not to provide testimony or documents in the House impeachment inquiry.
"The aggressive position he (Trump) took in the impeachment trial could have an indirect impact in the courts," said Michael Stern, a former House lawyer when it was under Republican control.
Trump lost in the lower courts in all three of the Supreme Court cases. Two involve Trump's bid to ward off congressional subpoenas issued to third parties - Trump's accounting firm Mazars LLP and two banks, Deutsche Bank AG and Capital One Financial Corp - seeking his bank records, tax returns and other material.
The other case involves a criminal investigation into Trump and his family real estate business in which Manhattan District Attorney Cyrus Vance, a Democrat, is seeking the president's tax returns. The Supreme Court is due to rule by the end of June in the three cases.
The Justice Department has argued that the House Judiciary Committee lacks legal standing to enforce its subpoena on McGahn and that senior presidential advisers are "absolutely immune" from being forced to testify to Congress about official acts. It also argued that federal courts lack the authority to referee such disputes between the legislative and executive branches of the U.S. government.
Another important legal fight was put on hold by a federal judge awaiting the McGahn case's outcome. In that case, the House Ways and Means Committee sued the Treasury Department to force it to hand over years of Trump's individual and business federal tax returns.
(Reporting by Lawrence Hurley and Jan Wolfe; Editing by Will Dunham)