Rep. Steve Knight, R-Calif. in Simi Valley, Calif., May 26, 2018. Bill Clark | CQ-Roll Call Group | Getty Images

GOP Rep. Steve Knight is in a fight for his political life, as the incumbent enters the final month of the midterm election campaign. His Democratic rival Katie Hill has a slight lead in the tightly contested congressional race in Southern California. And immigration, taxes and health care could be among the deciding factors in this closely watched race. In a sign of the his vulnerability, Knight has been distancing himself from some of President Donald Trump's policies, particularly when it comes to immigration. The at-risk Republican represents a district with a nearly 40 percent Latino population and has been critical of Trump's use of the pen for executive orders involving immigration as well as the administration's policy of separating children from their parents at the border. Hill, a former head of a nonprofit organization that helps homeless people, was relatively unknown when she announced her candidacy in early 2017 to unseat Knight in California's 25th Congressional District. The 31-year-old Democrat has been able to harness the collective power of women's organizations and a grassroots campaign to build greater awareness in the working-class district.

Katie Hill, Democrat running for California's 25th Congressional district seat in Congress, speaks during the opening of the SCV Democratic Headquarters for 2018 in Newhall, Calif., on Saturday, May 26, 2018. Bill Clark | CQ-Roll Call | Getty Images

The GOP has a 23-seat majority in the House, but Democrats are hoping to flip enough Republican-held seats in California and other close contests around the nation to wrest back control of the chamber they lost in the 2006 midterm elections.

'Must-win district'

In California alone, the Democratic party is targeting at least 10 GOP-held seats, including seven in the southern and central portion of the state that presidential candidate Hillary Clinton carried in 2016. Even Hill concedes the 25th district is a "must-win district" for the Democratic party. "There's no path to Democrats taking back the House that doesn't run through this district," Hill told CNBC in a recent interview. "We know this is going to be an election that comes down the wire. We've got millions of dollars being spent against us."

Knight, 51, is seeking a third term in the district and won reelection with about 53 percent of the vote back in 2016. The incumbent's campaign declined an interview request.

Poll: Hill leading Knight

A Los Angeles Times/University of California-Berkeley poll released last month shows Hill leading Knight 50 percent to 46 in the district located in northern Los Angeles County and eastern Ventura County. About 650 likely voters were surveyed in the poll, which is within the margin of error of 5 percentage points. "When you're an incumbent member of Congress and you're under 50 percent, that's never great," said Lawrence Becker, a political science professor at California State University-Northridge. "The district has historically been Republican but has been trending demographically toward the Democrats." A debate scheduled Oct. 25 could be critical for the candidates given how close the race is at this stage. That said, mail-in ballots for the November election started getting delivered this week so it's possible some voters won't wait until the debate. Two nonpartisan trackers of House races — the Cook Political Report and Sabato's Crystal Ball at the University of Virginia — both rate the Knight-Hill contest as a "toss-up." FiveThirtyEight, meanwhile, gives Hill a 7 in 10 chance of winning the seat from Knight.

Knight war chest

Hill has outraised and outspent Knight during the campaign. She generated about $2.4 million and as of Sept. 30 had less than $900,000 in the bank. Knight raised nearly $1.7 million and still has more than $1.3 million in the campaign war chest.

With less than a month before the election, Republicans are redoubling efforts to turn back a possible blue tide. The Congressional Leadership Fund, a Republican super PAC, is running ads for Republican candidates in at least five districts in California, including the 25th district. Three ads from the PAC seek to boost Knight's chances, including one knocking Hill as a "liberal" who supports policies that are "hurting middle-class families." The 25th district stretches from Palmdale and Lancaster in the east to Santa Clarita and the northern part of the San Fernando Valley to conservative Simi Valley — home to the Ronald Reagan Presidential Library. The Democrats hold a nearly 4-point lead over Republicans in voter registration in the district, although another important factor will be the nearly 94,000 who registered with "no party preference."

Trouble for GOP

"When you think of the big population centers of the district, such as Santa Clarita and Valencia, what comes to mind is upscale suburbs or exurbs," said Becker. "They are not only places where you have pretty high median income. They are also places where you have a lot of college-educated voters, which is the kind of district that has been giving Republicans some trouble this year." The political scientist added that independent and Republican voters with higher education levels are increasingly "turning against Trump." According to the website FiveThirtyEight, Knight has voted with Trump 98.9 percent of the time.

Immigration

Immigration is a top issue in the district and California's so-called sanctuary state policies that seek to protect undocumented immigrants have been opposed by some cities within the district, including Simi Valley and Santa Clarita. Knight, whose wife Lily is a Chilean immigrant, has sought to distance himself from Trump, particularly on immigration matters. For one, he criticized the Trump administration for its controversial policy of separating children at the border and introduced legislation in June on the issue. The congressman also has taken issue with the president's use of executive orders on immigration matters. "They've been very hard on immigration. They've been pushing ... raids and controls," Knight told the Santa Clarita Valley Signal in May. "I think it should [go] through Congress." Knight previously indicated he backs a legal residency for so-called dreamers but stopped short of embracing a pathway to citizenship. Trump wants to get rid of the Obama-era Deferred Action for Childhood Arrivals, or DACA, the policy that protects immigrants brought unlawfully to the U.S. as children from deportation and provides them work authorization. A California federal court order in April prevented the Trump administration from temporarily removing DACA protections. "Steve Knight says that he supports DACA recipients, but when push comes to shove he has done nothing to stand up to his party to make that happen," Hill said. "His wife is an immigrant, and so he leans on that a lot to say he's pro-immigration reform. But when you look at his record, it's actually quite the opposite." For her part, Hill has called on Congress to end the uncertainty for dreamers by passing a legislative solution. California has a lot at stake on the issue given about one fourth of the DACA recipients are in the state. Hill, who lives on a farm for rescue animals with her husband, has identified as bisexual and made LGBTQ rights a part of her platform.

Gun violence

At the same time, the Democrat has made gun violence an issue the campaign. It follows a mass shooting last year by a 14-year-old student at a high school in Palmdale. Hill, the daughter of a police officer, has run online ads touting how she's a "lifelong gun owner" and supporter of the Second Amendment. Nonetheless, she also supports a federal ban on assault weapons. Knight, an Army veteran who spent 18 years on the force of the LA Police Department, has been the target of ads from a pro-gun control super PAC started by former Arizona Rep. Gabrielle Giffords. One PAC ad accuses him of "selling out" to the gun lobby. He received donations from the National Rifle Association and has an "A" rating from the group. Regardless, Republicans are fighting back with their own barrage of ads.

Gas tax fight

The CLF super PAC released an attack ad last month that calls Hill "immature" and "out of touch." It also said she "supports radical regulations that would increase gas prices." "Katie Hill's support for radical regulations that would increase gas prices by another 60 cents shows how completely out of touch she is with California's middle-class families," said Michael Byerly, a spokesman for the CLF. Among other things, the Republican super PAC seeks to pin Hill as someone supported by proponents of California regulations such as cap-and-trade and an increase in the state's gasoline taxes. Last year, Democrats in the state legislature approved a 40 percent hike in the state's excise gas tax to help fund highway and street maintenance. A statewide gas tax repeal measure known as Proposition 6 is on November ballot, which could boost Republican candidates in the California's midterms. To be clear, Hill hasn't formally taken a position on the state gas tax increase but was endorsed by labor groups fighting the repeal effort. "No misleading ads or politics-as-usual tactics from outside super PACs are going to distract from the fact that our district is hungry for a representative who will prioritize them, not special interests or corporate donors," said Lindsay Bubar, a senior adviser to the Hill for Congress campaign. "This community knows Katie and they know she's going to be a fighter for everyone in this district, especially middle class families." Hill's campaign has fired back with its own TV ads.

Health care

In a recent ad, Hill appears to take a jab at Knight and "politicians who don't stand for anything besides their party and their corporate donors." Among other things, the ad promises she will push for "lower healthcare and prescription drug costs" as well as to "protect Social Security and Medicare." Last year, Knight voted for the health-care law change designed to repeal and replace parts of the Affordable Care Act (ACA) or ObamaCare. He also supported the American Health Care Act of 2017, which the nonpartisan Congressional Budget Office warned could lead to more than 23 million Americans losing health insurance The incumbent held at least three town halls last year, including one in Santa Clarita where he faced a fiery crowd and questions about why he voted to repeal ObamaCare. He responded by predicting ACA will fail to be a sustainable, affordable or accessible program in four year's time.

Federal tax reform