Young lawmakers who want new blood in the Democratic Party aren't rushing into the looming generational battle between Massachusetts Sen. Ed Markey, 73, and Rep. Joe Kennedy III, the 38-year-old millennial who's flirting with a primary challenge against him.

Insurgent Democratic Reps. Rashida Tlaib (Mich.), 43, Seth Moulton (Mass.), 40, and Eric Swalwell (Calif.), 38, told The Hill they're staying out of the race. And some young progressives, such as 42-year-old Rep. Ro Khanna (Calif.), who ousted veteran Democrat Mike Honda in a 2016 primary, are fully in Markey's corner.

On Friday, freshman Rep. Alexandria Ocasio-Cortez (D-N.Y.), the 29-year-old liberal firebrand with 5.3 million Twitter followers, threw her weight behind Markey, who introduced the Senate version of her Green New Deal measure earlier this year.

"Ed Markey is a proud and strong progressive champion for working families, not just in Massachusetts but across the country. And his leadership in authoring the Green New Deal, along with me, and carrying it in the United States Senate is emblematic of the kind of work that he does for working families across the country," Ocasio-Cortez said in a video endorsement.

"And Ed Markey was one of the few people that had the courage to stand up and take a chance. And take a chance on a freshman congresswoman. And take a chance on this plan," added Ocasio-Cortez, who ousted Rep. Joe Crowley (D-N.Y.) during a 2018 primary in one of the biggest political upsets in recent years.

Sources close to Ocasio-Cortez said she huddled with Kennedy on Thursday and warned him not to challenge Markey. Instead, she encouraged him to run for Sen. Elizabeth Warren's seat if she wins the White House next year or moves on from the Senate, the sources said.

Ocasio-Cortez and Markey also have been discussing the idea of holding a series of Green New Deal town halls throughout Massachusetts, a source said, the first of which would be later this year.

A handful of Massachusetts Democrats said they expect Kennedy to formally launch his bid against Markey any day now, though they added that Kennedy - the grandson of Robert F. Kennedy and a scion of one of America's greatest political dynasties - is keeping things close to the vest.

Asked by The Hill about his timetable for making a decision on the Senate, Kennedy replied, "As long as it takes to get there - and hopefully soon."

A source close to Kennedy says the four-term congressman has received plenty of encouragement from House colleagues, particularly younger members. But so far, only one lawmaker has publicly encouraged Kennedy to take on Markey: freshman Sen. Kyrsten Sinema (D-Ariz.), who befriended Kennedy when they were elected to the House in 2012.

Since then, they've held several joint fundraisers and even led a charity bike ride.

"Joe Kennedy is one of the most talented, committed people I have ever had the pleasure of knowing and serving with. He's got a vision and he gets things done," Sinema, 43, told The Hill. "He will be a tremendous senator."

The Kennedy-Markey fight comes amid Democratic tensions about generational change and whether younger blood is needed in the top echelons of the party.

After winning back the House majority last fall, Speaker Nancy Pelosi (D-Calif.), Majority Leader Steny Hoyer (D-Md.) and Majority Whip James Clyburn (D-S.C.) - all in their 70s at the time - fended off attacks by a band of young insurgents hungry for new leadership.

The issue of age was also front and center at Thursday's Democratic presidential debate in Houston, where former Housing and Urban Development Secretary Julián Castro repeatedly accused former Vice President Joe Biden of muddling an explanation of his health care plan. "Are you forgetting what you said two minutes ago? Are you forgetting already what you said just two minutes ago?" he asked.

Biden is 76, while his two closest rivals - Sen. Bernie Sanders (I-Vt.) and Warren - are 78 and 70, respectively.

Warren, the progressive presidential hopeful on the rise, has already endorsed Markey.

When it comes to Kennedy and Markey, lawmakers are not taking sides along traditional generational fault lines. Instead, the potential primary is likely to focus on which candidate is more progressive.

Markey declined to speak to The Hill about a potential challenge from Kennedy. But the senator's allies highlighted his role in crafting the Green New Deal and noted that he supported "Medicare for All" and the legalization of marijuana much earlier than Kennedy did.

"Markey is the leader on climate change and has been more progressive on Medicare for All, on legalizing marijuana, on taking on Big Pharma," Khanna, a progressive leader from Silicon Valley, told The Hill. "He will have the overwhelming support of the progressive base because of his record and also of many tech leaders given his leadership on net neutrality."

Kennedy and his team appear unfazed. A Suffolk University/Boston Globe survey published this month found that in a head-to-head match-up, 42 percent of likely Massachusetts voters said they would favor Kennedy, compared to Markey's 28 percent.

"As Congressman Kennedy considers the race he is focused on feedback from the people of Massachusetts, and them alone," Kennedy spokeswoman Emily Kaufman said in an email.

Both Markey and Kennedy are slated to appear Saturday at the Massachusetts Democratic Party's annual convention in Springfield, according to The Boston Globe.

Kennedy's moves against Markey, who served in the House from 1976 to 2013, are infuriating many senior Democrats who served with Markey and have bought into the system of seniority.

Those Kennedy critics say that the grandnephew of former President Kennedy is in a rush to win a Senate seat so he can run for president in 2024 or 2028. Some Democrats have speculated that Kennedy believes he has a better shot at taking out Markey, a fellow white male, than beating Rep. Ayanna Pressley (D-Mass), 45, the first black woman elected to Congress from the Bay State, in a future Senate race to replace Warren.

"This is F'd up. You're going to take on Ed Markey because you think he has an expiration date because he's in his 70s and you're not?" said one powerful female senior Democrat in the House.

"He's looking at running for president, and he knows he can't do that from the House and doesn't want Markey to go six more years. He's in his 30s!" the Democratic lawmaker said. "I just think it is incredible hypocrisy for the Democrats to decide the expiration date of people."

A second senior House Democrat added, "Those in my generation, we believe in waiting our turn, getting experience under our belt. And those younger generations do not embrace that; they live for the moment, and they are looking for instant gratification."

Tlaib, who along with Ocasio-Cortez and Pressley is a member of the so-called "squad" of four progressive freshman congresswomen of color, said neither she nor her Michigan constituents are focused on the potential Massachusetts race.

Swalwell, who ousted 40-year incumbent Rep. Pete Stark (D-Calif.) in a 2012 primary and repeatedly challenged Biden to "pass the torch" to a new generation during a presidential primary debate, had no comment about a potential Kennedy-Markey fight.

And Moulton, who ousted incumbent Rep. John Tierney (D-Mass.) in a 2014 primary, ran for president this year and then ended his campaign months later, is also staying neutral for now.

"I'm just listening," he said. "I'm not saying one way or the other."

But other fresh faces in Congress said primary races are the new normal in the Democratic Party and that they are keeping their options open as Kennedy nears his decision.

"There is a strong case to be made that we are going to have more of these races where it is about a generational change, where it's about bringing new voices. We have seen some of that happen in the House; quite frankly we have not seen much of it in the Senate," said freshman Rep. Katie Hill (D-Calif.), who is a junior member of Pelosi's leadership team.

"I'm one of the people who believes this is the Democratic process and we shouldn't be opposed to primaries happening," she added.