WASHINGTON (Reuters) - At least three U.S. House of Representatives Democrats were preparing to sign a letter protesting a plan to speed up the official party approval of President Joe Biden's reelection bid, the lawmakers' offices said on Tuesday.

These Democrats are protesting a move by the party to hold a "virtual roll call" vote on Biden becoming the nominee as soon as July 21, instead of awaiting the Democratic National Convention Aug. 19-22 in Chicago.

Democratic Representatives Susan Wild, Mike Quigley and Jared Huffman plan to sign the letter, representative of each lawmaker said when contacted by Reuters.

"Stifling debate and prematurely shutting down any possible change in the Democratic ticket through an unnecessary and unprecedented 'virtual roll call' in the days ahead is a terrible idea," said a copy of the draft letter seen by Reuters. "It could deeply undermine the morale and unity of Democrats."

The effort is separate from that of the 19 congressional Democrats who have called on Biden, 81, to end his campaign after a halting June 27 debate performance against Republican challenger Donald Trump, but marks continued unrest in the incumbent's party about his campaign.

U.S. Representative Adam Schiff, a California Democrat running for his state's Senate seat, who was not one of the 19, warned donors in a private meeting that his party would likely suffer major losses if Biden continued his reelection bid, the New York Times reported on Tuesday. A spokesperson for Schiff's campaign declined to comment.

Last month's debate raised concerns in the party about both Biden's ability to beat Trump and also his fitness to hold the high-pressure job for another four years.

Thirty-nine percent of Democratic respondents to a Reuters/Ipsos poll completed on Tuesday said they believed that Biden should end his White House run, a slightly higher reading than the 32% who said that in a Reuters/Ipsos poll days after the debate.

The letter has not yet been sent to the DNC and was being circulated widely among House Democrats, according to congressional sources.

Democrats fear that a poor performance by Biden in the Nov. 5 election could cost their party not only control of the White House but both chambers of Congress, setting the stage for a second Trump administration that would be able to pursue its policy goals with almost no Democratic opposition.

Republicans followed their party's standard procedure in officially nominating Trump at their convention in Milwaukee on Monday.

There has been speculation that if Biden were to drop out of his re-election campaign, the Democratic Party might coalesce around Vice President Kamala Harris as its presidential nominee.

Some Democrats, however, could insist on a more open process that would allow other potential candidates to vie for the nomination, less than three months before the general election.

There had been some concerns that the Democratic convention in August to crown a nominee was coming too late to have that nominee on Ohio's ballot.

The letter to the DNC argued that Ohio has enacted legislation to negate that obstacle, making the virtual roll-call vote in July unnecessary.

(Reporting by Richard Cowan; Additional reporting by Rami Ayyub and Makini Brice; Editing by Scott Malone and Alistair Bell)

By Richard Cowan