A Democratic super PAC is making in a second six-figure investment on a television advertising buy in the Massachusetts Senate race, a person familiar with the buy confirms, a move that comes with Election Day fast approaching and polls showing Democratic Rep. Ed Markey's GOP opponent within striking distance.

U.S. Senate candidate Ed Markey shakes hands with a supporter in Boston, Tuesday, April 30, 2013 as he celebrates winning the Democratic primary for the special U.S. Senate election. Markey and Republican former Navy SEAL Gabriel Gomez won their party primaries Tuesday, setting up a race between a 36-year veteran of Washington politics and a political newcomer for the U.S. Senate seat formerly held by John Kerry. (AP Photo/Elise Amendola) U.S. Senate candidate Ed Markey (AP Photo/Elise Amendola)

Senate Majority PAC is adding $500,000 to its existing television ad buy. The group hit the airwaves last week with a spot that hits Republican nominee Gabriel Gomez over his position on Social Security. The super PAC initially put about $500,000 behind the negative ad.

The latest financial reinforcement comes with just 13 days before the special election for the seat once held by Secretary of State John Kerry. A recently conducted Suffolk University survey shows Gomez trailing, but within striking distance of the Democrat, who held a 48 percent to 41 percent advantage.

National Democrats have been making a big stretch-run push on behalf of Markey, while Republican groups have much been quieter so far. The Democratic Senatorial Campaign Committee has hit the airwaves, Vice President Biden has raised money for Markey, and President Obama will stump for the congressman later Wednesday.

-- Aaron Blake contributed to the post