That's up five percentage points from last year, and it's the highest level since 2003, when 65 percent of Americans expressed approval. The all-time low polling for union approval, as uncovered by Gallup, was in 2009 at the beginning of the Obama Administration, when union approval was at 48 percent. It was the only time in the history of Gallup polling that the union approval number dipped below 50 percent.

"Unions have regained the approval of the American people after dropping during the Great Recession," Gallup's Art Swift said in the article accompanying the release of the numbers. "As more time passes since the bailout of two of the Big Three auto companies, a possible reason unions dipped in approval, it appears that unions are once again solidly popular.

"There are likely limits to this approval, however. Republicans' lower approval of unions, as part of a growing political polarization on a number of issues, means that solid union support may never return to the levels seen from the 1930s to 1960s."

Gallup said 81 percent of Democrats approve of unions in 2017 - significantly higher than the 42 percent of Republicans who approve. That's less of a gap than it was in 2011, when Republican approval was 26 percent and Democratic approval was 78 percent. 

"Democratic approval of unions has been fairly steady over time, while the approval levels of independents and Republicans have fluctuated," Gallup's Swift says. "Republicans' approval of unions rose since last year, possibly due to the presidency of Republican Donald Trump. Even though Trump is not an avid supporter of unions, his rhetoric about restoring U.S. manufacturing jobs and cordial relations with some top labor union leaders at the start of his term may have softened Republican attitudes about unions."

He added: "Republican approval of unions is similar to when the last Republican president, George W. Bush, left office. It is possible that Republicans may now perceive unions as less threatening because Trump is unlikely to expand their power."

Historically, unions have enjoyed "strong support" from the American public, Gallup says. That support was strongest in 1936, when there was a 72 percent approval rating, and in the mid-1950s, when approval soared to 75 percent. 

With greater approval of unions comes more acceptance of unions gaining more influence. Gallup reports that 39 percent of Americans would like unions to have more influence -- the highest figure recorded in the 18 years Gallup has asked this question. Still, Gallup said Americans remain more pessimistic than optimistic about unions' future. Forty-six percent say they think unions will become weaker than they are today, while 27 percent say they will be the same and 22 percent say stronger.