It's not the first time Democrats have used this attack on Rehberg, but the ad is significant for mentioning the Buffett Rule by name at a time when the debate over the rule has largely fallen to the wayside.

Though the rule, named for the billionaire investor Warren Buffett, who proposed that his own tax rate be higher than it is currently, was a large part of the messaging push for Democrats engaging in the tax reform debate this past spring, of late Medicare, rather than tax policy, has taken center stage.

But the DSCC seems to think it could be a potent issue in Montana, where Rehberg is running to take the seat from incumbent Sen. Jon TesterJonathan (Jon) TesterDemocrat Manchin: Pence attacks prove ‘they don't want bipartisanship’ in Trump admin Tester invited the Border Patrol Union’s president to the State of the Union. What does that say to Dreamers?   These Democrats will have a hard time keeping their seats in 2018 MORE, in a close race — and one that both sides think could be a key win to control the Senate. 

However, Rehberg's campaign seemed confident that the attack wouldn't hold water because, as campaign spokesman Chris Bond said, "Tester supports President Obama's tax-and-spend agenda 95% of the time."

"So if liberal Tester's out-of-state allies want to waste millions of dollars trying to convince people to trust Tester and Obama over Rehberg and Romney when it comes to lowering taxes, that's just fine with us," he said.

Watch the ad:

--This post was published at 4:19 on Sept. 5 and updated at 10:20 on Sept. 6.