Intense pressure by major campaign contributors and heavyweight lobbying combined led up to today’s strange-bedfellow vote by the Senate Energy and Natural Resources Committee to advance a bill greenlighting the Keystone XL pipeline.

Senate Energy and Natural Resources Committee Chairman Mary Landrieu, D-La., the author of the bill, had to rely on the support of 10 Republicans — whose party leadership is busy trying to defeat her this fall — to win approval of her measure. Landrieu and Sen. Joe Manchin of West Virginia were the only two Democrats voting with the majority when the panel voted, 12-10, in favor of the bill.

Landrieu’s insistence on moving ahead with the pipeline puts her at odds with her party: President Barack Obama’s administration, while so far not opposing its construction, is reviewing its environmental impact, but he’s under intense pressure from environmentalists to block it. Landrieu, however, who is one of the most vulnerable Democrats up for re-election this year, is hoping her advocacy will help her with her oil-patch constituents.

The National Republican Senatorial Committee is trying to preempt that with the ad above that questions whether Landrieu has the clout to move her party’s leaders. It’s part of a blitz of ads from outside groups trying to influence the debate over the pipeline. As you can see on Sunlight’s Political Ad Hawk, many target vulnerable Democrats like Landrieu. Hers is one of several Democratic seats the GOP hopes to turn this year in order to win control of the Senate.

The Keystone XL pipeline, which would stretch from Canadian tar sands in Alberta all the way to refineries on the Gulf of Mexico, has prompted a lengthy influence battle involving lobbyists, TV ads, campaign cash and grassroots organizing.

MORE: Read all of Sunlight’s coverage on the behind the scenes battle for Keystone XL

The battle lines are complicated. While Landrieu is being targeted by the NRSC, other traditional GOP allies, such as the American Petroleum Institute, have commended the senator’s efforts to move forward a Keystone vote. API chipped in $5,000 to Landrieu’s campaign committee, data from OpenSecrets.org shows and has pressured the other vulnerable senators on the energy committee in a series of TV ads.

Contracts picked up by Political Ad Sleuth reveal that, in May alone, the oil industry trade association purchased more than $260,000 worth of air time in the Washington market.

On Capitol Hill, the pipeline has attracted lobbyists from a myriad of energy producers and environmentalist groups.

Congressional lobbying registration data compiled by Sunlight Foundation’s Influence Explorer shows that Keystone XL has been on the agendas of a wide range of organizations, running the gamut from labor unions to petroleum companies to environmental groups.

But the pace of lobbying appears to have picked up in the first quarter of 2014. According to a search of records at the Senate Office of Public Records, more than 50 organizations reported mentioning Keystone XL in the first quarter of the year. See the full list we turned up here. (Note that the “amount reported” is the organization’s entire lobbying budget for the first three months of the year, not necessarily the money spent lobbying on Keystone XL)

Among opponents: the League of Conservation Voters, the League of Women Voters and the National Resources Defense Council. But supporters have a much higher historic influence profile. They include major multinational oil companies such as Exxon Mobil and Chevron, trade associations such as API and the American Fuel and Petrochemical Manufacturers and the National Association of Manufacturers, as well as labor unions, which are touting the pipeline as a jobs creator.

In addition, lobbying disclosures by foreign clients to the Department of Justice, also available on Influence Explorer, show that the Canadian province of Saskatchewan had extensive contacts last year with members of Congress that will play key roles in determining the fate of the pipeline, and worked with Texas Gov. Rick Perry to place a pro-Keystone XL op-ed in The Hill newspaper, widely read in the halls of Congress.

Similarly, lobbying powerhouse Mehlman, Vogel, Castagnetti reported “relationship-building” contacts with key members of Congress and their staffers on behalf of the province of Alberta.