The 2012 photos show Maine candidates for U.S. Senate in the November 2012 general election, Democrat Cynthia Dill, left, Independent Angus King, center, and Republican Charlie Summers. (AP)

With GOP candidates struggling in some key states, the U.S. Chamber of Commerce is doubling its effort to boost the Republican candidate in Maine’s three-way Senate race with another advertisement against former governor Angus King and a slight nod to also helping the Democratic nominee.

This is the third ad that the chamber has run against the independent King, who has not announced which party he would caucus with if elected but is widely expected to side with Democrats. The chamber accused King of raising taxes and spending during his eight years as governor, ending in 2002.

“More spending, higher taxes and a budget shortfall — fact is, King was a major disappointment,” the narrator says in the ad.

The latest volley from the chamber, which has endorsed Maine Secretary of State Charlie Summers (R), comes as recent polls have shown that King’s once formidable lead has shrunk. Republicans saw this three-way race as an opportunity in which they could win by getting just 40 percent of the vote, as happened in the state’s 2010 gubernatorial campaign, when Republican Paul LePage emerged from a three-way race to win with just 38 percent of the vote.

This time around, Republicans believe they need to see state Sen. Cynthia Dill (D) get up to 20 percent or more of the vote, and the new round of chamber ads have a new slight political nod that might be meant to drive Democratic voters away from King and into Dill’s corner.

“When King left office, Democratic Governor Baldacci had to dig out of a billion-dollar budget hole,” the ad’s narrator says, referring to King’s successor, John Baldacci.

However, these ads come as GOP candidates in other Senate races with major backing from the chamber and other conservative groups have floundered.

Combined with national GOP leaders abandoning Missouri and races tightening in Republican-held seats in Indiana and Arizona, Senate Republicans have an increasingly narrow path to picking up the four-seat gain that they need to claim the majority for the first time in six years.

This has forced Republicans and their allies such as the chamber to look at Maine as a seat they could steal from Democrats, who cheered when three-term Sen. Olympia Snowe (R-Maine) announced her plan to retire, denouncing partisanship on her way out the door. National Democrats abandoned their party’s nominee, Dill, privately believing that King’s popular stewardship of the state in the late 1990s and post-partisan message were a perfect fit for the times.

King has defended his term as governor, suggesting that his spending programs were done efficiently. At a debate this week, he boasted of a program to give laptops to middle school students, calling it a “key and tool for the 21st century.”