When Joe Biden Joe BidenOmar fires back at Trump over rally remarks: 'This is my country' Trump mocks Biden appearance, mask use ahead of first debate Trump attacks Omar for criticizing US: 'How did you do where you came from?' MORE enters the presidential race next month, it’s not a given that the former vice president will have a ton of support from his old Senate colleagues.

Biden served in the Senate for 36 years, but his time only overlaps with 18 Democrats who are still in the upper chamber. Nearly 30 Democrats, most a generation younger than Biden, have never served a day with the former chairman of the Judiciary and Foreign Relations panels.

He’ll also be squaring off against a half dozen current senators — including Kamala Harris Kamala HarrisButtigieg stands in as Pence for Harris's debate practice First presidential debate to cover coronavirus, Supreme Court Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (Calif.), Cory Booker Cory Anthony BookerBipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE (N.J.), Elizabeth Warren Elizabeth WarrenHarris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda Judd Gregg: The Kamala threat — the Californiaization of America GOP set to release controversial Biden report MORE (Mass.) and Bernie Sanders Bernie SandersButtigieg stands in as Pence for Harris's debate practice Bernie Sanders warns of 'nightmare scenario' if Trump refuses election results Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (I-Vt.) — who have forged their own relationships in the upper chamber.

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“It’s a different Senate today. The challenge for him will be there are so many new senators and where that center of gravity lands, I think it’s too early to tell,” said Sen. Martin Heinrich Martin Trevor HeinrichSenate Democrats demand White House fire controversial head of public lands agency Senate Democrats seek removal of controversial public lands head after nomination withdrawal Five takeaways from final Senate Intel Russia report MORE (D-N.M.), who arrived in the Senate in 2013, four years after Biden had left to become vice president.

“I have a lot of respect for him,” said Heinrich, who is 47. “My bias is that we have this new, incredible generation of leadership and that in 2020, my hope is that the race will reflect that and I would like to see the nominee reflect that.”

To date, Biden has secured endorsements from three Democratic senators: Biden’s two home-state senators, Tom Carper Thomas (Tom) Richard CarperDemocrat asks for probe of EPA's use of politically appointed lawyers Overnight Energy: Study links coronavirus mortality to air pollution exposure | Low-income, minority households pay more for utilities: report OVERNIGHT ENERGY: Democrats push resolution to battle climate change, sluggish economy and racial injustice | Senators reach compromise on greenhouse gas amendment stalling energy bill | Trump courts Florida voters with offshore drilling moratorium MORE and Christopher Coons Christopher (Chris) Andrew CoonsMurkowski: Supreme Court nominee should not be taken up before election Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE of Delaware, plus Dianne Feinstein Dianne Emiel FeinsteinBiden leads Trump by 12 points among Catholic voters: poll Names to watch as Trump picks Ginsburg replacement on Supreme Court McConnell says Trump nominee to replace Ginsburg will get Senate vote MORE (Calif.), who served with Biden on the Judiciary Committee when he was chairman. Freshman Sen. Doug Jones (Ala.) also has been publicly encouraging Biden to run.

All four senators have lavished praise on the former vice president even though he still hasn’t officially entered the race. In January, Feinstein went as far as saying: “My candidate would be Joe Biden.”

But many others on Capitol Hill are not rushing to endorse Biden, who could face the same problems that plagued then-New York Sen. Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE in 2008 when she was the front-runner against Barack Obama Barack Hussein ObamaMichelle Obama and Jennifer Lopez exchange Ginsburg memories Pence defends Trump's 'obligation' to nominate new Supreme Court justice The militia menace MORE.

Back then, Clinton was taken aback when her Senate colleagues — many of whom she had known for years — turned their backs on her and supported Obama, at the time a senator from Illinois. The toughest blow for Clinton may have been when the late Sen. Ted Kennedy (Mass.) endorsed Obama, but she felt betrayed by other senators like Claire McCaskill Claire Conner McCaskillMomentum growing among Republicans for Supreme Court vote before Election Day Democratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally MORE (Mo.), who chose to support Obama instead.

The 76-year-old Biden could be put in a similar situation.

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“There may be quite a few senators who vote with their hearts,” said one longtime Senate aide, adding that it will be an “uphill climb” for Biden.

Sen. Richard Blumenthal (D-Conn.) said he and Biden are “friends” but that he has not heard from the vice president in awhile.

“You know what, I don’t know,” Blumenthal said when asked if he plans to endorse anyone.

The other senator from Connecticut, Democrat Chris Murphy Christopher (Chris) Scott MurphyDemocratic senator calls for 'more flexible' medical supply chain to counter pandemics The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon GOP chairman to release interim report on Biden probe 'in about a week' MORE, campaigned with Biden in Hartford last fall and worked with him on gun control in the wake of the Sandy Hook Elementary School mass shooting in 2012. But with so many current Senate colleagues running, Murphy said he’ll probably steer clear of an endorsement.

“I have great admiration for him. I think everybody who joins the field makes it better and stronger,” Murphy said of Biden. “He brings a national security background that I think will challenge the rest of the field to step up their game.

But he added: “I have lots of friends running, so I would be foolish to wade in. It’s a no-brainer for me.”

Biden has been in touch with members of the Senate.

Murphy said he recently spoke with Biden on some policy matters but did not discuss the presidential race. Sen. Bob Casey Robert (Bob) Patrick CaseySecond GOP senator to quarantine after exposure to coronavirus GAO report finds brokers offered false info on coverage for pre-existing conditions Catholic group launches .7M campaign against Biden targeting swing-state voters MORE Jr. (D-Pa.), who is close to Biden and represents the state where he was born, has been in contact with him in recent weeks. And Sen. Jon Tester Jonathan (Jon) TesterPence seeks to boost Daines in critical Montana Senate race This World Suicide Prevention Day, let's recommit to protecting the lives of our veterans Filibuster fight looms if Democrats retake Senate MORE (D-Mont.), who briefly overlapped with Biden in the Senate, said he called his former colleague two weeks ago to catch up on his likely White House bid and other issues.

Asked if he was encouraging Biden to run, Tester replied: “I certainly would not discourage him. Joe’s a good friend.”

The change in the Senate “has been generational,” Tester said, “but he still has a lot of old friends.”

Across the Capitol in the House, support for Biden has been hit or miss. Behind the scenes, he’s been reaching out to a select group of lawmakers and asking for their endorsements.

Biden’s longtime adviser Steve Richetti also has been making calls to shore up support. Ricchetti even invited Rep. Don Beyer, the former Virginia lieutenant governor and a key House fundraiser, over to his home for dinner recently, though Beyer described the evening as two old friends catching up rather than a Biden pitch for president.

But the Biden calls and invites aren’t going out to everyone.

House Oversight and Reform Committee Chairman Elijah Cummings Elijah Eugene CummingsBlack GOP candidate accuses Behar of wearing black face in heated interview Overnight Health Care: US won't join global coronavirus vaccine initiative | Federal panel lays out initial priorities for COVID-19 vaccine distribution | NIH panel: 'Insufficient data' to show treatment touted by Trump works House Oversight Democrats to subpoena AbbVie in drug pricing probe MORE (D-Md.) has known Biden for decades but said he hasn’t spoken to him “in years.” Another veteran Democrat, Rep. Lacy Clay William (Lacy) Lacy ClayFive things we learned from this year's primaries Progressives aim for big night in Massachusetts Progressives look to unseat top Democrat in Massachusetts primary MORE (Mo.), hasn’t heard directly from Biden either, though he’s been in touch with two other 2020 hopefuls, Harris and Booker, his colleagues in the Congressional Black Caucus (CBC).

“They serve in the Congress with me and in the CBC, so I am considering both of them,” Clay told The Hill. “They both seem to be doing pretty well out there on the campaign trail as far as attracting crowds, speaking to the issues that Americans care about.”

One of Biden’s strongest arguments in a Democratic primary is the idea that he can beat President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE in states such as Michigan and Pennsylvania that turned away from Democrats in 2016.

It’s not hard to imagine Biden being a favored candidate among House lawmakers representing swing districts that Trump won in the last presidential election.

At the same time, there’s reason for House liberals to think twice about backing Biden, who supported the Iraq War and whose chairmanship of the Anita Hill hearings during Justice Clarence Thomas's confirmation process has come under new scrutiny.

Rep. Lois Frankel Lois Jane FrankelShakespeare Theatre Company goes virtual for 'Will on the Hill...or Won't They?' Florida Democrat introduces bill to recognize Puerto Rico statehood referendum Hillicon Valley: Democrats demand answers over Russian interference bulletin | Google Cloud wins defense contract for cancer research | Cyberattack disrupts virtual classes MORE (D-Fla.), co-chair of the bipartisan Women’s Caucus, said she’ll be backing a woman in 2020, though she hasn’t decided which one.

“In my heart … we have so many good women running for president, I would like to see one of them make it,” Frankel told The Hill.

“Kamala Harris, Kirsten Gillibrand Kirsten GillibrandSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Suburban moms are going to decide the 2020 election Jon Stewart urges Congress to help veterans exposed to burn pits MORE, [Minnesota Sen.] Amy Klobuchar Amy KlobucharSocial media platforms put muscle into National Voter Registration Day Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE, Elizabeth Warren — I would support any of them. Tulsi’s running too,” she said, referring to Rep. Tulsi Gabbard Tulsi GabbardRepublicans call on DOJ to investigate Netflix over 'Cuties' film Hispanic Caucus campaign arm endorses slate of non-Hispanic candidates Gabbard says she 'was not invited to participate in any way' in Democratic convention MORE (D-Hawaii).

Some Biden allies said they don’t put too much stock in the congressional endorsement game. Clinton had overwhelming support among her former Senate colleagues in 2016, yet Sanders mounted a stronger-than-expected challenge in the primary that year, Carper recalled.

And Hill surrogates aren’t always on message. A close Biden friend for four decades, Carper played a practical joke on a reporter from The Hill who asked whether Biden was close to jumping in.

“I heard this morning that he’s getting cold feet and that his wife has had second thoughts. And you know what they say in the Biden household: Happy wife, happy life,” Carper said with a serious expression. “I expect him to announce that he’s not going to run.”

“Are you joking?” the reporter replied.

A wide smile appeared on Carper’s face: “It’s not a joke; it’s an outright lie.”

The senior senator from Delaware said he knows the entire gaggle of senators running for president very well, adding that there are no bad feelings given his early Biden endorsement.

“In that gathering,” Carper said, “there are a lot of terrific potential vice presidential candidates.”