WASHINGTON (Reuters) - U.S. congressional Democrats, worried that President Joe Biden's campaign troubles will hurt their chances of capturing a House majority in the Nov. 5 election, return to Washington on Monday to try to agree on whether to urge the incumbent to end his reelection bid.

A handful of House of Representatives Democrats have already called on Biden, 81, to terminate a campaign that has been on defense since a shaky June 27 debate against Republican Donald Trump. A series of public events since then have not ended Democrats' questions about whether Biden will be able to win or hold up through another four years in power.

Democrats face an uphill battle to defend their 51-49 Senate majority, with incumbents up for reelection in multiple Republican-leaning states, and some in the party view recapturing a House majority as their best chance of keeping a hold on one of the levers of power in Washington should Trump, 78, win.

Biden has repeatedly insisted that he plans to remain in the race, saying he is the best choice to beat Trump, who once again has not committed to accepting the election's outcome, and Biden views as a risk to America's democratic history.

Biden will continue to visit churches, union halls and other venues to meet voters in coming days, administration officials said. At the same time he plans to reach out to lawmakers he has known for decades, they said, as he tries to calm concerns.

A growing number of Democratic lawmakers have voiced concern that his poor public approval, including concerns about his age and ability, could hurt the party in the roughly two dozen most competitive House races. Vice President Kamala Harris is seen as the likeliest successor should Biden step aside.

"At the present, our down-ballot candidates in the Senate and House are doing well. They're all ahead. They are running well ahead of the president, but you can only run so far ahead," Democratic U.S. Representative Adam Schiff, said on NBC on Sunday.

"So, obviously, Joe Biden is going to need to consider, for his own sake and his own legacy, can he beat Donald Trump? Is he the best to beat Donald Trump?" said Schiff, who is running for Senate in California. "But also the profound impact this decision, maybe the most important of his presidency, will have on the House, on the Senate, on the future of the country."

Schiff and congressional Democrats including Representative Jim Clyburn have also voiced confidence in Harris.

Hardline Republican Representative Chip Roy last week proposed the House vote on a resolution calling on Harris to invoke a provision of the U.S. Constitution's 25th Amendment that allows the Cabinet to vote to install the vice president as acting president if the actual president is unable to carry out his duties.

Late in Trump's presidency following the Jan. 6 2021 attack on the U.S. Capitol, the U.S. House of Representatives voted to urge Vice President Mike Pence to start the 25th Amendment process of removing the president from office.

Biden on Sunday made a series of campaign appearances in Pennsylvania, a battleground state, aimed at easing Democrats' worries.

THE AGE FACTOR

A Reuters/Ipsos poll last week found that one in three Democratic registered voters believed that Biden should quit the race, with 59% of respondents in the president's party saying he is too old to work in government.

U.S. Senator Mark Warner was aiming to convene a meeting of fellow Democrats in the Senate on Monday -- where Biden served for 36 years -- to discuss his campaign, a source familiar with his plans told Reuters in recent days. It was unclear how many senators planned to participate, and some reports late Sunday said those plans had fallen apart.

House Democrats were also weighing support for draft letters urging Biden to end his candidacy.

Biden's troubles appear to be increasing the number of races Democrats need to worry about in November.

Internal party polling shows that New Mexico and Virginia became more competitive following the debate, according to a source familiar with the findings, and the nonpartisan Center for Politics at the University of Virginia last week shifted its ratings on the states of Michigan and Minnesota to make each slightly more favorable for Republicans.

Together, those states will host a half-dozen of the most competitive House races, a meaningful number given that Republicans currently control the chamber by a narrow 219-213 margin.

Republicans' 19 months in the majority have been marked by chaos, including the unprecedented ouster of their former Speaker, Kevin McCarthy. Both McCarthy and his successor, Mike Johnson, have repeatedly had to turn to Democrats for the votes to pass critical legislation, including bills to keep the government open and avert a historic and catastrophic default on the nation's debt.

If Republicans were to capture the White House and both houses of Congress, Trump would face few constraints on his ability to push through major policy changes.

Democrats may still have a hard time capturing the House if they fail to hold the presidency.

"It would be historically odd if Democrats flipped the House while losing the White House at the same time," said Kyle Kondik, managing editor of "Sabato's Crystal Ball" at the University of Virginia. "Any erosion for Democrats in the presidential race likely has a negative effect on the race for the House."

(Reporting by Moira Warburton and Richard Cowan, additional reporting by Andrea Shalal; Editing by Scott Malone and Bill Berkrot)

By Moira Warburton and Richard Cowan