In front of the same bright yellow bus at another rally in Southern California two days later, surrounded by similar bright yellow signs, Republican Rep. Mimi Walters, who’s in a tight race in Orange County, denounced liberals for raising taxes.

“The first step in taking back our state is repealing the gas tax!” Walters tweeted that day.

Days before the midterm elections, California Republicans are calling upon a common line of attack line against Democrats: taxes. The bus tour, dubbed the Rush Hour Rally, is designed to stoke outrage about a recently passed law that raises vehicle fees and gas taxes. Proposition 6, the ballot measure to repeal it, is the rallying cry. Even as recent polls show that the measure is losing ground among voters in the Democrat-dominated state, proponents insist the opposite as they travel from city to city, armed with bright, eye-catching visuals to hammer their message.

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They’re hoping Prop 6 fulfills a common goal of ballot measures: energizing a party’s base. It lets California conservatives talk about a salient issue that invigorates Republicans without invoking President Trump, said GOP political consultant Mike Murphy.

“There’s a thinking that, hey if the gas tax becomes a big state issue, it’ll help the right over the left,” Murphy said.

For Walters and other vulnerable congressional Republicans running in suburban Southern California districts that Hillary Clinton won in 2016, the measure is a way to appeal to college-educated women who normally vote Republican but dislike Trump, Murphy said.

“There are two things that Prop 6 potentially offers. One is the idea of getting out to vote in Southern California,” said Bill Whalen, a Hoover Institution research fellow.

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The second, he said, is reintroducing the idea of taxation in a state that has raised taxes twice via ballot measures in the past decade. In a state where driving is ingrained in daily life, a driver would pay about $750 annually in fuel taxes and vehicle fees as a result of the tax hike, according to the state’s Legislative Analyst’s Office.

“You can’t run on Donald Trump, you cannot run on the tax cut, you cannot run on trade wars,” Whalen said. “What do you have left to talk about? What is out there to get Republicans angry?”

In North Dakota, there are two initiatives on red-meat issues: Ballot Measure 3, which would legalize marijuana, and Ballot Measure 2, which would clarify that only U.S. citizens and state residents can vote in elections. Backers of each say they weren’t created to stoke turnout.

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But the marijuana measure, initiated by the grass-roots group Legalize N.D., is likely to attract younger voters who tend to vote Democratic and may help incumbent Sen. Heidi Heitkamp (D), although she didn’t endorse the measure, said John Hudak, a Brookings Institution senior fellow. The other initiative, the brainchild of a former state GOP chairman, tries to outlaw something that’s already illegal and won’t have a significant policy impact. Still, it’s a catnip for hard-line conservatives.

In North Carolina, a ballot measure would create a constitutional right to hunt, fish and harvest wildlife. It’s backed by the National Rifle Association and is another draw for conservatives, but critics say it unnecessarily tries to protect something that’s not under threat.

The key is not just rallying behind a hot-button issue. Also important is picking a cause that one side cares deeply about and on which the other side is less organized, Hudak said.

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In 2004, for example, the national Republican Party mobilized several states to pass ballot measures banning same-sex marriage — and to boost Republican turnout in key states that reelected President George W. Bush. In 2016, ballot measures to raise the minimum wage were seen as a way to boost Democratic turnout.

Measures that push turnout are even more necessary during midterm elections, when there aren’t presidential campaigns around which parties can mobilize nationally, said Caroline Tolbert, a University of Iowa political science professor who has studied the effects of ballot initiatives on voter turnout.

“There’s quite a bit of evidence — our work was some of the earliest work that shows it — that turnout is higher when you have these salient initiatives on the ballot. And it has to do with how salient they are,” Tolbert said.

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Those behind the measures, however, insist they’re not defined by partisan politics.

Cole Haymond, a consultant working with grass-roots activists to legalize marijuana in North Dakota, said he has talked to voters who support the measure and Heitkamp’s Republican opponent in the Senate race, Rep. Kevin Cramer. Others, he said, are dismayed with national politics, but will vote on local races and on the ballot measure.

“I would certainly hope that everyone exercises their right to vote, and I think in North Dakota, Measure 3 will turn out people that, like I said, are not traditional voters or nonvoting potential voters,” Haymond said. “What I can say is that if it’s a low turnout, it’s not just over for Heidi; it’s over for us.”

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Gary Emineth, the former chairman of the North Dakota Republican Party who’s also running for the state Senate, said he spearheaded the measure on illegal voting after learning of instances in which non-U.S. citizens voted in local elections. Nationally, there has been no proof of widespread voter fraud by undocumented immigrants. But the issue has been used by Republicans from President Trump on down to appeal to the party’s base.

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“I’m doing it because it’s the right thing to do,” Emineth said of the ballot measure.

In North Dakota, the Senate race is already riling up so many voters that there isn’t much turnout left to boost, said University of North Dakota political science professor Mark Jendrysik.

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Carl DeMaio, a former San Diego city councilman and the Proposition 6 campaign leader, bristled at the notion that the measure was meant to boost turnout and help vulnerable Republicans. It’s “foul and misleading and completely illegitimate,” said DeMaio, a Republican and conservative radio host. He added that Proposition 6 is a citizen initiative without party affiliation.

“Are there Republicans supporting it? Yes, great," he said.

Some elected officials did see Proposition 6 as a way to spur voter support amid a real threat of Democratic takeover of the House. House Speaker Paul D. Ryan (R-Wis.), House Majority Leader Kevin McCarthy (R-Calif.) and Walters have each poured thousands of dollars from their campaign coffers to help the repeal effort. So did Cox — whom polls have shown trailing Democratic nominee Lt. Gov. Gavin Newsom by double digits.

But the excitement about Proposition 6 has begun to falter, as powerful infrastructure and transportation groups put millions of dollars toward defeating the measure. An October survey by the nonpartisan Public Policy Institute of California found that 41 percent of likely voters said they would vote “yes” on the measure, while 48 percent would vote “no,” choosing to keep the tax hike.

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“It was an attempt to change the conversation for congressional candidates, from Donald Trump to the gas tax. So far, it hasn’t worked,” said Robert Shrum, a former Democratic strategist who runs the Jesse M. Unruh Institute of Politics at the University of Southern California. “Trump continues to be the elephant in the polling place. People are voting for him or against him.”

Using Proposition 6 as a way to appeal to conservative voters may not be working for key Republicans, as well. Democrat Katie Porter, who’s seeking to unseat Walters, announced in an August television ad that she opposes higher gas taxes.

Other Democrats in swing districts also have departed from their party, with some explicitly saying they support Proposition 6. These defections seem to neutralize the issue, Shrum said.

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“There’s still some chance the proposition could pass, but it’s very long odds,” Shrum said. “It hasn’t proven to be the silver bullet that Republicans thought it would be.”

Cleve R. Wootson contributed to this report.