By Trevor Hunnicutt

NEW YORK (Reuters) - Two of the three Republicans challenging Donald Trump for their party's presidential nomination backed an effort to impeach the U.S. president during a debate on Tuesday.

Republicans Joe Walsh and Bill Weld made the comments as the U.S. House of Representatives planned to launch a formal impeachment inquiry into Trump over reports he sought Ukrainian help to smear Democratic presidential front-runner Joe Biden.

"The Ukraine caper by the president is some combination of treason, bribery and other high crimes and misdemeanors," said Weld, a former Massachusetts governor. "The one thing that's absolutely clear is it is grounds for removal from office."

Walsh, a radio talk-show host and former U.S. congressman from Illinois, agreed.

"Donald Trump is unfit," he said during a debate hosted by the website Business Insider. "The president of the United States will deserve to be impeached very soon."

The event in New York was not sanctioned by the Republican Party, which has thrown its full support behind Trump. He did not participate and also called the impeachment inquiry "Witch Hunt garbage" on Twitter.

Trump has dismissed his challengers as a "joke" and "laughingstock."

Asked to explain Trump's decision not to participate in the debate, Erin Perrine, a deputy communications director for the Trump campaign, said, "Pointless."

Former South Carolina governor and congressman Mark Sanford, the third Republican challenging Trump, said he missed the debate because of a scheduling conflict.

Walsh, Weld and Sanford have complained about being shut out of the nomination contest. Republican Party officials in Alaska, Arizona, Kansas, Nevada and South Carolina already have said they would not host nominating contests in their states next year, cutting off chances for the challengers to win delegates who will choose the Republican candidate at the party's convention. The Republican nominee will face the Democratic Party's candidate in the November 2020 election.

A Sept. 16-20 Reuters/Ipsos poll showed that 82% of registered Republicans approved of Trump's job performance, while a similar poll this week found that 37% of the American public thinks Trump should be impeached, down from 41% in a similar poll earlier in September.

Walsh, 57, served in the House of Representatives for two years and then became a radio talk-show host. Weld, 74, touts a fiscally conservative record as well as his work on gay rights and environmental issues. He ran unsuccessfully for vice president in 2016 on the Libertarian Party's ticket.

(Reporting by Trevor Hunnicutt; Additional reporting by Chris Kahn; Editing by Soyoung Kim, Will Dunham and Cynthia Osterman)