Kamala Harris waves upon arrival at Joint Base Andrews in Maryland. / Stephanie Scarbrough/Pool via Reuters

WASHINGTON (Reuters) -Vice President Kamala Harris' move to the top of the Democratic ticket injected new life into her party's flailing campaign against Republican Donald Trump, but some lawmakers whose support she will need if she wins are keeping her at arm's length.

In the 35 competitive races that will determine control of the 435-seat U.S. House of Representatives in the Nov. 5 election, 12 Democratic incumbents or challengers so far have held off endorsing Harris.

They are presenting themselves as independent decision-makers focused on their constituents rather than national politics, distancing themselves from a candidate that Republicans tie to the Biden administration's record on immigration and inflation.

"Our choice in leaders should never be a knee-jerk decision driven solely by party loyalty," said Representative Jared Golden of Maine, one of the 12, who said he wanted to see what policies Harris would propose that might help citizens of his district, before deciding whether to support her.

Golden was also one of six House Democrats who voted last week in support of a Republican resolution condemning President Joe Biden's administration - and Harris specifically - over the administration's handling of migrants crossing the southern border with Mexico. They hailed from Alaska, Colorado, Maine, North Carolina, Texas and Washington state.

Just one of those states - North Carolina - is expected to be competitive in the November election. But all six districts will play an important role in determining control of the House.

Republicans currently have a narrow 220-212 majority and if Democrats win control of the House, they could either help drive Harris' agenda forward if she wins or serve as a bulwark against former President Trump if he prevails.

Golden and the other Democrats who joined in voting for the Republican resolution stand in a distinct minority within the Democratic ranks in Congress.

The broader Democratic Party has rallied quickly behind Harris since Biden abandoned his reelection bid and endorsed her on July 21.

Harris' campaign said it swept up more than $200 million in campaign donations and signed up 170,000 volunteers in the first week. Opinion polls showed her closing the gap on Trump and her early momentum has reshaped the race, which could help down-ballot candidates by driving up turnout and enthusiasm.

"Her background as a prosecutor is something that really appeals to people who are concerned about rising crime," said Representative Hillary Scholten, whose Michigan district is one of the 35 most competitive post-primary races.

"They're concerned about law and order and that's something that, you know, I'm really hearing from people that is appealing to them," Scholten said. "I'm talking about people who have literally never voted for a Democrat except for me."

Scholten, who had publicly called on Biden to end his campaign, said she has campaigned in her district alongside Harris, adding, "We invite her back."

Scholten was one of 41 Democratic House women who joined former Speaker Nancy Pelosi last week to endorse Harris' candidacy.

HOLDING OFF ON ENDORSEMENTS

Colorado Democrat Adam Frisch, who is seeking the seat currently held by firebrand Republican Lauren Boebert, is among the challengers whose success would be critical to the party capturing a majority in the House.

He has not endorsed Harris and like most candidates in competitive districts, is seeking to highlight his independence on policy issues. He was one of the first Democratic candidates to call for Biden to end his campaign.

"Working Colorado families are fed up with career politicians who aren’t focused on the issues that matter to them, and both parties are at fault," said Frisch, who will face Republican Jeff Hurd in November as Boebert runs in a more conservative district.

Biden, 81, agreed to end his campaign under pressure from Democrats, including 36 in Congress who publicly urged him to do so, saying they feared he would hurt their electoral prospects in the House and Senate.

Candidates in closely contested districts have reason to keep their distance from their party's presidential nominees, said Larry Sabato, head of the University of Virginia's Center for Politics.

"It took some in-party pushing and hard feelings to get Biden out. So now they’re supposed to play nice and treat their struggle to keep a House seat in tough territory as patty-cake? No way," Sabato added.

Sometimes, they must walk a fine line. Representative Matt Cartwright, of Pennsylvania, another of the 35 Democrats locked in a close House race, endorsed Harris but sought to keep his distance by focusing on local issues.

"I don't spend a whole lot of time talking about the White House," Cartwright told Reuters. "My allegiance is to the people of northeast Pennsylvania. I don't care who they vote for president."

A source familiar with the Harris' campaign's thinking said that all Democratic candidates should do what is needed to succeed.

"That’s the first priority, and they want candidates to run the races they need to run to win – as is typical for any election cycle," said the source, who was granted anonymity to discuss internal deliberations.

NEED FOR SUPPORT

Presidential candidates have always made big promises, as Harris and Trump are doing now. Harris is pledging to sign a law making abortion legal across the nation while Trump has vowed to extend his 2017 tax cuts and offer fresh tax breaks, such as on tipped income.

Neither will be able to deliver on those promises without sufficient support in Congress. Trump's and Biden's major legislative victories came during their first two years in office when their parties held majorities in both the House and Senate. Each saw priorities stymied when they lost full control of Congress in midterm elections.

Democrats face a more difficult path to a majority in the Senate, where one in three seats are up for grabs and they must defend seats in strongly Republican states including Montana, Ohio and West Virginia.

Supporters say Harris has brought new energy into the race after Biden struggled and ultimately failed to overcome serious concerns about his age and mental acuity.

She has also excited many Democrats by more forcefully advocating for abortion rights and going after Trump over his criminal convictions in a hush-money trial in New York as well as charges he faces in other cases.

"Led by Kamala Harris, Democrats have an incredible agenda to run on," Harris campaign spokesperson Kevin Munoz said in an email. "This election, whether it’s top of ticket or a state election, will be decided on the issues that matter most to voters, which is why Democrats will win this November."

Representative Abigail Spanberger, whose Virginia seat is also on the most-competitive list, said her office was already receiving calls from constituents asking how to get tickets to a potential Harris inauguration.

"There's excitement and so that excitement translates on the ground," said Spanberger, who is now running for governor of Virginia.