In a new email to supporters, Markey said the "chained CPI" proposal in Obama's budget could be thought of as standing for "Cutting Peoples' Income."

"This is a wrong-headed move. Too many seniors — almost two-thirds, in fact — rely on Social Security for at least half of their income. Budgeting requires compromise, but we cannot compromise by cutting Social Security," he says in the email.

Markey asks supporters to sign a petition calling for Obama to abandon the proposal, which he characterizes as "roll[ing] over to the right wing." 

The congressman was one of more than 80 House Democrats to sign onto a resolution rejecting the proposal, which remains controversial among Democrats.

The email knocking Obama's plan represents an interesting calculation for Markey, who is running in a state where the president won more than 60 percent support in 2012.

But chained CPI is one area where Markey hopes to characterize his opponent, Republican Gabriel Gomez, as being on the wrong side of an issue.

Gomez has expressed support for the proposal. Markey's campaign has attacked Gomez on it over the past week, characterizing him as being in favor of cuts to Social Security.

Chained CPI is a formula for inflation that would likely result in cuts to government benefits and tax increases over time. Supporters of the formula say it is more accurate than the one now used to calculate Social Security benefits. 

Gomez hit back in a statement chargin that Markey, with his 1993 vote for President Bill ClintonBill ClintonCongress needs to assert the war power against a dangerous president House Dems push to censure Trump over Charlottesville response Too many Americans with insurance are being denied coverage MORE's budget, supported cuts to Social Security benefits for seniors.

Ed MarkeyEd MarkeySenate Dem: Trump has to stop ‘reckless’ language on North Korea Trump sparks debate over war resolution for North Korea Foreign Relations Dem: North Korea is the modern-day Cuban missile crisis MORE has spent almost 40 years in Washington, and has done nothing to save the future of Social Security. In fact, he voted to cut seniors’ Social Security checks. It’s time to retire Ed Markey, and protect our seniors’ future," he said.

Markey's email is also, in part, an effort to collect contact information on the far-left flank of Democratic voters in Massachusetts.

Markey will need to turn those voters out to the polls to ensure a win. More than 35 percent of the state's voters are registered Democrats, but more than half of them are independents. Gomez is making a play for independents and conservative Democrats to try and replicate former Sen. Scott Brown's surprise win in 2010.

Markey will need a comfortable cushion of progressive Democratic supporters if independents break for Gomez.

A recent poll from Democratic firm Public Policy Polling gave Gomez a lead among independent voters, taking 47 percent support to 31 percent support for Markey. That same survey showed Markey up by only four percentage points.

--This piece was updated at 2:06 p.m. to reflect comment from Gomez's campaign.