In Texas’ 21st congressional district, walking from street to street, knocking on doors and asking for votes is not as straightforward as it sounds.

The district covers parts of the state’s liberal heartland in central Austin, dips down to the north-eastern suburbs of San Antonio and protracts about 200 miles west into remote and ultra-conservative cowboy country.

The district’s shape is so contorted by gerrymandering that when Mary Wilson canvasses in urban areas of Austin and San Antonio, she sometimes has to get out a map to check which side of the boundary she’s on.

But there is another dividing line, next to which Wilson stands on the left: a Democratic divide between progressives and moderates that will be closely watched on Tuesday, as Texas holds party runoff elections ahead of November’s midterms.

Democrats need to flip 23 seats to take control of the House and were it not for the way Austin is carved up, the 21st would be a likely victory rather than an intriguing outside bet.

The city is covered by six districts that extend far out into conservative strongholds, diluting the influence of Travis County, where Hillary Clinton won 66% of the vote in 2016. Austin residents comprise a minority of each district’s population; five of the six are in Republican hands.

How did I, this lifelong Democrat, become that ‘anti-establishment’ candidate? Mary Wilson

In a year when the Democratic base is highly energised and the 21st’s Republican incumbent, Lamar Smith, is retiring, the district is a test case for whether Democratic prospects are best served by a candidate who can inspire exceptional turnout from liberals or one that could woo moderate crossover Republicans, disturbed by the party’s direction under Donald Trump.

Wilson, a minister and former math teacher, believes that the Washington establishment, in the form of the Democratic Congressional Campaign Committee (DCCC), favours her runoff rival, Joseph Kopser, a decorated army veteran and entrepreneur with endorsements from a number of party figures. The DCCC has not officially endorsed a candidate in the race.

“How did I, this lifelong Democrat, become that ‘anti-establishment’ candidate?” she asked. “Well, I became that because the DCCC decided they liked my opponent’s story, they thought somehow he can win, and lo and behold he [raised] nearly $800,000 on a primary and came in second and didn’t even break 30% of the vote.

“I don’t know why then we should consider him to be the most viable candidate to win in November when he couldn’t win in March with every advantage. It just doesn’t make any sense to me. I think quite frankly what that really points to is what the voters want is something different than what the DCCC has picked.”

Despite being a virtual unknown who raised only $40,000, Wilson finished ahead of Kopser in the primary. “I think that we are in an election cycle in the United States right now where voters want more female voices,” she said. “Being a progressive-minded person, social justice advocate – and being a female social justice advocate – really fits the tenor of the mood in the country right now.”

If Wilson wins on Tuesday her path to victory in November will require a high turnout in Austin and San Antonio. Kopser believes he can persuade substantial numbers of Republicans to cross sides in a district represented for three decades by Smith, a notorious climate science sceptic and favourite of the fossil fuel industry who is chairman of the House science, space and technology committee.

We just have to engage new voters out here Laura Moser

“You have two candidates, Mary and I, who line up very nicely on all the progressive issues we’ve been fighting for as a community for so long,” Kopser said, “whether rights of women, rights of immigrants, rights of workers, rights of the working middle class, protecting the environment, doing all of it. The only difference is we just have a very different set of backgrounds.”

His campaign, he argues, can “build a bridge of those independents and ‘orphaned’ Republicans, build a bridge of trust for them to know that we can give them the top cover they need to come over and vote for a Democrat”.

‘Unify the vote’

A similar battle is taking place to the east in the seventh congressional district, which covers some of the wealthiest parts of Houston. Home to Ted Cruz and George H W Bush, it first attracted national attention as a traditionally Republican area that could be ripe for a surprise result.

Then, in February, the DCCC released opposition research designed to discredit Laura Moser, fearing the anti-Trump activist and writer is too leftwing to stand a chance in November. The moved seemed to backfire by galvanising her campaign, which enjoyed a fundraising boost.

With no candidate securing a majority of the vote, Moser forced a runoff by finishing second in the primary to Lizzie Pannill Fletcher, a business attorney who has been endorsed by Emily’s List, an influential group that works to elect pro-abortion rights Democratic female candidates.

Moser’s campaign held a barbecue and rally in a Houston park on Sunday afternoon, with a “unify the vote” theme.

“Lizzie’s more centrist, moderate, middle of the road,” said Joshua Butler, who finished sixth in the Democratic primary and is now backing Moser. “Laura does have a little bit more of a left-leaning approach but I don’t think there’s anything wrong with that. At this stage right now we’ve got to have new ideas and different ways of getting things accomplished.”

The park is in a neighbourhood known as the energy corridor, home to many conservative-leaning white-collar oil and gas industry employees.

“This part of the district is not as Republican as the Republicans think, but we just have to engage new voters out here,” Moser said.

“I think this is a year where we really need politicians who are committed to changing the way we do business in Washington, who won’t just be like a safe vote, who won’t adhere to the status quo because it’s just not really working.”

