Democrats in Washington are tripling down on their bet that Katie McGinty is a better option than Joe Sestak to be their nominee to challenge Sen. Patrick J. Toomey (R-Pa.) this fall, launching a low-seven-figure campaign ahead of the April 26 primary.

The independent advertising arm of the Democratic Senatorial Campaign Committee has reserved $1.1 million worth of advertising across Pennsylvania in the final two weeks of the state’s primary, designed to boost establishment favorite McGinty, according to a media buyer who is closely tracking advertising in the heated contest.

Despite trailing in the polls, McGinty emerged as the preferred option for President Obama and the leadership team of Minority Leader Harry M. Reid (D-Nev.), who believe that Sestak, a former two-term congressman who narrowly lost to Toomey six years ago, might lose a seat critical for Democratic hopes of reclaiming the Senate majority.

McGinty, the former chief of staff to Pennsylvania Gov. Tom Wolf (D), has only one previous statewide race under her belt, falling far behind Wolf in the 2014 primary.

On Monday, Vice President Biden, visiting Pittsburgh to deliver a speech about combating sexual assault on college campuses, greeted McGinty at the city’s airport and then made an unannounced stop at a local diner to pitch her candidacy inside vote-rich Allegheny County, the second-largest bloc of votes in a Democratic primary behind Philadelphia in the state’s southeastern corner.

Sestak, a former admiral with close ties to former President Bill Clinton, has taken the shunning from the establishment in stride, trying to turn it into a positive in this anti-establishment era.

The DSCC declined to comment on the ad buy, as did representatives for the firm that is handling independent expenditures for the Pennsylvania primary.

The late surge of DSCC cash comes amid some signs that McGinty’s campaign might finally be breaking through, after many months of languishing far behind the better-known Sestak. A Republican polling company released a survey Monday that showed Sestak with support from 41 percent of the likely voters in the race, with McGinty at 31 percent. John Fetterman, mayor of Braddock, a small city in western Pennsylvania, received 9 percent in the survey by Harper Polling.

The good news for the DSCC was McGinty’s movement — she rose 14 percent in one month in the Harper poll. The bad news was that Sestak also rose, up from 33 percent a month ago.

Still, the gap narrowed, and the Obama-Biden endorsement is just settling into voters’ minds — something that McGinty’s allies will work aggressively to inform the undecided voters in the campaign’s final days. The president and vice president remain very popular with Pennsylvania Democrats, with almost two-thirds saying that they have a “very favorable” image of the two party leaders.

The DSCC’s ad buy comes on top of a nearly $1 million investment by Women Vote, a super PAC that is an arm of Emily’s List, a liberal group devoted to electing women.