Rep. Joe Kennedy III, left, elbow-bumps Sen. Ed Markey after their Democratic Senate primary debate on June 1 in Springfield, Mass.

Matthew J. Lee/The Boston Globe via AP/Pool

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Matthew J. Lee/The Boston Globe via AP/Pool

Rep. Joe Kennedy III, left, elbow-bumps Sen. Ed Markey after their Democratic Senate primary debate on June 1 in Springfield, Mass.

Matthew J. Lee/The Boston Globe via AP/Pool

Massachusetts voters cast ballots Tuesday in one of the last state primaries of the year.

While many voters will head to the polls in person, most people planning to cast ballots in the primaries have already voted, according to Massachusetts officials.

On Monday, Secretary of State William Galvin told reporters that hundreds of thousands of ballots had already been cast, either by mail or through early voting.

There are two key races to watch in Massachusetts, and both feature a younger Democratic challenger taking on an older incumbent.

Senate (Democratic) — Markey vs. Kennedy

Since 2018, several prominent longtime Democratic politicians have faced competitive reelection challenges from younger, more progressive opponents.

The Massachusetts Senate primary veers from that narrative, because Sen. Ed Markey, who’s 74, is the candidate who’s garnered progressive support in his high-profile campaign against Rep. Joe Kennedy III, though the 39-year-old shares similar positions.

Markey has served in the Senate since 2013 and was a congressman for 37 years before that. He co-authored the Green New Deal with New York progressive Rep. Alexandria Ocasio-Cortez, who supports his campaign. Markey has also been endorsed by his colleague, Sen. Elizabeth Warren.

Kennedy, who also supports the Green New Deal, is the grandson of Robert F. Kennedy, and has served in Congress since 2013. In late August, he was endorsed by House Speaker Nancy Pelosi of California.

Some Democrats, including former Massachusetts Rep. Barney Frank, criticize Kennedy for seeking a higher political position during the 2020 election season. Frank told WBUR’s Anthony Brooks last year that Democrats should work to unseat Republicans, saying diverting resources from that effort is “a terrible mistake.”

Kennedy has criticized Markey’s record on racial justice. Markey opposed court-ordered busing to desegregate Boston Public Schools in the 1970s and voted for the 1994 federal crime bill that’s blamed for contributing for mass incarceration.

Despite being the challenger, Kennedy entered the race as the perceived favorite. But Markey now holds an 11-point lead, according to a polling average from Real Clear Politics.

Massachusetts has an all-Democratic congressional delegation, so it’s highly likely that the winner of the primary will go on win in the general election this November. (There are also competitive primaries to fill the House seat Kennedy is giving up.)

1st Congressional District (Democratic) — Neal vs. Morse

Progressives are hoping for an upset in Massachusetts’ 1st Congressional District as longtime Rep. Richard Neal faces a challenge from Holyoke Mayor Alex Morse.

Neal, 71, is a leader in the House of Representatives, serving as chair of the Ways and Means Committee. He has been in Congress for 32 years and has the endorsement of Pelosi as well as Massachusetts Gov. Charlie Baker, who is a Republican.

Morse, 31, was elected mayor of his native city at just 22. He has the support of Ocasio-Cortez, the climate group the Sunrise Movement and the Justice Democrats, a progressive political action committee. Morse is campaigning on key progressive issues including Medicare for All and the Green New Deal.

One recent issue in this race: vague allegations of impropriety against Morse, who’s openly gay, which led to an investigation of him by the University of Massachusetts. Additional revelations led Morse to say he’s been vindicated and, he says, have helped his campaign with donors and volunteers.