Rep. Edward Markey Edward (Ed) John MarkeyMassachusetts town clerk resigns after delays to primary vote count Bogeymen of the far left deserve a place in any Biden administration Senate Democrats urge Amazon to recall, stop sales of explosive products MORE (D) is heavily favored to win the Democratic primary Tuesday in the race to fill the Massachusetts Senate seat vacated by Secretary of State John Kerry John Forbes KerryThe Memo: Warning signs flash for Trump on debates Divided country, divided church TV ads favored Biden 2-1 in past month MORE — but the Republican contest remains far murkier.

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Markey has maintained a double-digit lead in polls over his opponent, Rep. Stephen Lynch (D), throughout a long campaign that was roiled by the Boston Marathon terrorist bombing and a budding Boston mayoral race.

The congressman is buoyed by a heavy fundraising advantage and a ground operation reminiscent of President Obama's. He also captured strong establishment support, including a late endorsement from Caroline Kennedy, daughter of former President Kennedy.

But Lynch has maintained a confident stance in public, telling The Boston Globe he has a 5-point lead over Markey and predicting he'll benefit from low turnout. Political observers in the state also warn that Massachusetts is known for its upsets, noting the 2010 special election when Republican Scott Brown was elected to the Senate.

There is considerably more uncertainty about the outcome of the Republican primary, with few polls available to take the pulse of the three-man field. About 200,000 voters are projected to cast ballots in the Republican primary, 35,000 more than in 2009.

The GOP field pits local businessman Gabriel Gomez against former ATF acting Director Michael Sullivan and state Rep. Dan Winslow.

Gomez has significant support from the Republican establishment in Massachusetts, backing that includes an endorsement from former Gov. William Weld. He also established a substantial fundraising lead in the first months of the campaign.

Winslow counts endorsements from every newspaper that has weighed in on the GOP primary, a fact he touted in a new ad launched Monday.

But he’s failed to gain traction in most polls of the race, and is considered an unlikely underdog.

Sullivan has run to the right of the other two candidates, touting his pro-life and pro-gun credentials, and has the support of a Tea Party group. That could help boost his turnout in a Republican primary, but could hinder him in the June 25 general election.

The Democratic candidate will enter the general election campaign as the heavy favorite to win. President Obama carried Massachusetts in 2012 with more than 60 percent support over Republican Mitt Romney, a former governor of the state.

A Markey nomination has seemed like a foregone conclusion since Kerry's (D) retirement on Feb. 1. The progressive dean of the Massachusetts delegation is pro-abortion rights and a leader on environmental issues. He supported President Obama’s healthcare law, which Lynch did not, a fact he frequently touted in debates.

An argument initially considered one of the strongest against him — an attack on his residency — hasn't picked up steam in the primary. Markey has spent most of his time in Maryland during his three-decade tenure in Congress.

For Lynch to win, he would need an enthusiastic turnout among blue-collar voters and among socially conservative but economically liberal Catholic voters, who make up a large portion of Massachusetts’s population.

The turnout battle hinges on which special interest groups can launch a stronger ground game in Massachusetts, as both Lynch and Markey count traditionally active grassroots groups behind them.

Markey has been endorsed by green groups, including the League of Conservation Voters and a group funded by California billionaire Tom Steyer, as well as pro-abortion rights groups like NARAL and Planned Parenthood.

All of those groups pledged to support Markey with substantial canvassing and get-out-the-vote operations, and Markey's own campaign has said it's previously knocked on 20,000 doors in a single weekend.

Lynch has the support of a number of local unions, long known for their organizing strength, particularly in Massachusetts, where strong union support helped boost Sen. Elizabeth Warren Elizabeth WarrenNo new taxes for the ultra rich — fix bad tax policy instead Democrats back away from quick reversal of Trump tax cuts It's time for newspapers to stop endorsing presidential candidates MORE (D-Mass.) to a win in 2012.

Lynch's campaign says it has been targeting diverse voters in urban areas, particularly in Markey's South Boston district. But his supporters will have to bring blue-collar voters out to the polls as well — many of whom reside in Markey's Malden-based district — if they hope to outperform Markey.

Secretary of State William Galvin (D) predicted about 550,000 voters would turnout for the Democratic primary, a substantial decline from the 669,000 voters that cast a ballot in the 2009. Galvin said that was a "generous" estimate, according to the Statehouse News Service.

Galvin cited the multiple high-profile news events over the past two months — including the Boston mayoral race that broke open when Mayor Tom Menino (D) announced his planned retirement and the Boston Marathon bombings — as reasons for the decline in enthusiasm for the Senate primary.

He also noted that Markey and Lynch, for all their efforts to establish contrasting narratives over the past two months, have struggled to differentiate themselves in the eyes of voters.

“Here in the Democratic side, you have two people, even though there may be some policy differences, who are remarkably similar in terms of background and outlook,” he said.

Indeed, the narratives established by both candidates at the outset — Markey as a progressive policymaker known for his leadership on Capitol Hill, Lynch as a populist fighter with working-class roots — were often overshadowed by fights over nuanced policy details during the candidates' televised debates.

The past week marked a more aggressive shift in Lynch’s campaign, however. He predicted several times that he would win.

Starting with two back-to-back debates held last week, Lynch came out on the offensive, attacking Markey's record on national security in the wake of the Boston Marathon bombings.

Mary Anne Marsh, a Massachusetts Democratic strategist, said Lynch's offense "may be too little too late."

“The Lynch I saw in those two debates last week was the Lynch I expected to see during this whole campaign and did not see,” she said.

The bombings mark a significant wildcard in the campaign. All the Democratic and Republican candidates took careful steps to avoid appearances of politicizing a tragedy, pausing their campaigns for the week after the incident. But they also shifted strategy in response to the bombings.

Lynch released a straight-to-camera ad thanking Boston police officers for their work during the crisis. Markey reissued an ad touting his legislative work on national security following the 9/11 terror attacks.

Gomez appeared in multiple press reports and television interviews discussing his experience during the marathon, which he ran and finished just a few minutes before the bombs went off. Sullivan gave a number of televised expert-testimony interviews.

But for all their efforts to harness any potential momentum out of the bombings, the event may ultimately prove to be more detrimental to the candidates’ chances, because the attack drew much of the attention the candidates have sought over the past two months.

The Democratic and Republican nominees will have less than two months following Tuesday’s primary to regain that attention in time for the June 25 general.