CNN Key Race Alert: This is the narrow path to a Democratic Senate

The President carried Indiana and Missouri by 19 points each, but a pair of new polls released this week by NBC News/Marist show Democratic Sens. Joe Donnelly and Claire McCaskill waging competitive re-election fights. In a two-way contest, Donnelly leads Republican Mike Braun 49% to 43% in the Hoosier State, while McCaskill and Josh Hawley are locked in a dead heat -- each with 47% -- in the Show Me State.

Another prime target for Republicans is Florida, where Quinnipiac University found Democratic Sen. Bill Nelson and Republican Gov. Rick Scott running even with 49% each after some $37 million in television ads -- roughly half of that by Scott's campaign.

The Blue Wall of Great Lakes states that Trump toppled in November 2016 so far looks to be holding for Democrats in 2018. Sens. Bob Casey (Pennsylvania), Sherrod Brown (Ohio), Debbie Stabenow (Michigan) and Tammy Baldwin (Wisconsin) are all favorites for re-election. Of that group, Baldwin appears to have the toughest race, facing a challenge from conservative state Sen. Leah Vukmir.

To be sure, the deep red states where Democratic incumbents are hoping to hang on may play to type and keep the majority out of reach for the party. But if a blue wave crests higher than expected, it could lift Democrats in states that would be all but lost in a more neutral environment.

Democrats are on offense in Nevada, a state Hillary Clinton carried by 2 points and where Rep. Jacky Rosen is now challenging GOP Sen. Dean Heller. The Battle Born State could be ground zero for the health care debate this cycle, with Rosen aggressively attacking Heller's support for the GOP's effort to repeal the Affordable Care Act.

Republicans breathed a sigh of relief late last month when GOP Rep. Martha McSally emerged from a tough primary contest in Arizona with a convincing victory against conservative firebrands Kelli Ward and Joe Arpaio. But McSally's shift to the right during the nominating fight, particularly on immigration, could pose a challenge in the general against Democratic Rep. Kyrsten Sinema, who entered the general election largely unscathed.

The list of Democratic targets this cycle has doubled, with a pair of red states -- Tennessee and Texas -- looking increasingly competitive. With the Senate currently split 51-49 in favor of Republicans, if Democrats were able to win either of those contests -- assuming they also flip Arizona and Nevada -- it would mean the party could afford to see one of its incumbents defeated and still preserve a path to the majority.

Tennessee Senate: Race moves from Lean Republican to Toss-Up

In Tennessee, former two-term Gov. Phil Bredesen is running a methodical campaign against GOP Rep. Marsha Blackburn by focusing on local issues and emphasizing his own moderate brand of politics over that of the national Democratic Party. That strategy seems to be working to his point, with an NBC News/Marist poll released Thursday showing Bredesen and Blackburn running neck and neck -- 48% to 46% -- among likely voters.

Bredesen's standing is buoyed by his popularity -- 61% of likely voters have a favorable view of the Democrat versus 22% who hold a negative view of him. By comparison, 46% of likely voters view Blackburn favorably compared with 36% who do not.

When it comes to the President, 47% approve of his job performance while 43% disapprove. Blackburn, a staunch ally of the President, is making the argument that electing Bredesen could give Democrats control of the Senate -- a persuasive message in a state that went for Trump by 26 points. Bredesen has pledged to work with the President when he proposes policies that benefit Tennessee, declaring at the start of his campaign he was not "running against" Trump.

Texas Senate: Race moves from Likely Republican to Lean Republican

Republicans are sending the cavalry to Texas for Sen. Ted Cruz, with the President announcing he'll do an October rally for his former 2016 GOP primary rival, and the political arm of the Club for Growth launching a seven-figure television ad campaign to hammer Cruz's Democratic opponent, Rep. Beto O'Rourke.

O'Rourke has been a fundraising phenom, more than doubling Cruz in the second quarter and leading the GOP incumbent in cash on hand by almost $5 million. Polls show O'Rourke within striking distance of Cruz. The latest NBC News/Marist survey showed the race at 49% to 45% in Cruz's favor.

For Democrats, the path to victory in the Lone Star State has been elusive. The party's last statewide win came in 1994. The last time Texas elected a Democrat to the Senate was 1988. Clinton lost the state by nine points -- an improvement on Mitt Romney's 16-point margin over Barack Obama in 2012. Having tapped into the Democrats' enthusiasm edge, O'Rourke appears poised to narrow the gap even more this year.

West Virginia Senate: Race moves from Toss-Up to Lean Democratic

West Virginia should be at or near the top of the GOP's targets this cycle based on Trump's 42-point margin there in 2016. The President has also visited the Mountaineer State six times since taking office.

The difficulty for Republicans is they are running against Democratic Sen. Joe Manchin, a popular two-term governor who has demonstrated a willingness to buck his party and support the President's policies and nominees. He voted for Trump's first Supreme Court selection, Neil Gorsuch, and has signaled he's open to supporting Brett Kavanaugh, saying Wednesday he has not seen anything disqualifying from the nominee during this week's Senate Judiciary Committee hearings. Manchin also has demonstrated he can win in a less-than-ideal environment, receiving 61% of the vote in 2012 even as Romney carried the state with 62% support.

Republican nominee Patrick Morrisey, the state's attorney general, has faced questions about his fundraising ability and Manchin has a six-to-one cash advantage. Morrisey only recently launched his first television ad, a biographical spot that features praise from the President and highlights his legal challenge to environmental regulations implemented by the Obama administration.

That comes after Democratic outside groups have spent the summer pummeling Morrisey on TV over his work as a "DC lobbyist" for pharmaceutical companies and his decision to join a lawsuit challenging the ACA, a move that could lead to protections for pre-existing conditions being struck down. That line of attack could resonate in West Virginia, which at 36% has the highest rate of adults under 65 with pre-existing conditions, according to a Kaiser Family Foundation study in 2016.

North Dakota Senate: Race moves from Toss-Up to Lean Republican

One state where Republicans are increasingly bullish about their chances is North Dakota, where GOP Rep. Kevin Cramer is challenging Democratic Sen. Heidi Heitkamp in a state the President carried by 36 points. Trump was personally involved in the effort to recruit Cramer into the race and stumped for the three-term GOP lawmaker Friday night in Fargo.

For a while it appeared Heitkamp was Trump's favorite Senate Democrat. The President praised her as a "good woman" during a September 2017 event in the Peace Garden State and invited her to the White House in May for the signing of a bank deregulation bill she co-sponsored.

Now it seems the President is all-in for Cramer. "You need a senator who doesn't just talk like they're from North Dakota, but votes like they're from North Dakota. That's what you need, and that's Kevin Cramer," Trump said during a Fargo rally in late June.

Heitkamp should be able to keep the race close because of her strong personal brand and resource advantage -- leading Cramer $5.2 to $2.4 million in cash on hand. Cramer's bottom line should get a boost thanks to the President's visit, which his campaign says raised over $1 million.

Democrats are attacking Cramer on health care, accusing him of voting to strip protections for people with pre-existing conditions, and trade policy, a particularly acute concern in agriculture-rich North Dakota. Republicans, meanwhile, are targeting Heitkamp on hot-button issues such as abortion and so-called sanctuary cities, as well as her vote against the GOP tax cuts.

Given Trump's popularity in the state, Cramer's unwavering support for the President could be a more potent pitch to voters than Heitkamp's message of moderation. The national environment this year favors Democrats, but North Dakota is very much Trump country -- and Cramer is Trump's candidate.

New Jersey Senate: Race moves from Solid Democratic to Likely Democratic

It's been nearly 50 years since New Jersey voters elected a Republican to the Senate.

Bob Hugin is a longshot to break that streak, but the pharmaceutical executive is giving Democratic Sen. Bob Menendez a serious challenge in the Garden State. A recent Quinnipiac University poll showed Hugin trailing Menendez by six points in a state Clinton won by 14 points in 2016 and Obama carried by 17 points four years earlier.

The federal corruption trial involving Menendez ended last November in a mistrial, but an ethical cloud continues to hang over the Democrat. Nearly half of New Jersey voters say Menendez was involved in serious wrongdoing, including 38% of Democrats.

Democratic primary voters in the state delivered a clear message to Menendez in June when little-known challenger Lisa McCormick drew 38% of the vote against the two-term incumbent. Still, Menendez received more votes than were cast in the GOP primary, which Hugin won with 75% support.

Hugin has loaned his campaign more than $15 million and has spent more than $7 million on television ads to boost his candidacy. Menendez has been saving his resources -- holding about $6.4 million cash on hand. He's also getting a little help from the Democratic Senatorial Campaign Committee, which recently teamed with Menendez on a joint television ad buy. Democrats feel confident Menendez's numbers will improve once he hits the airwaves. He also stands to benefit from several competitive House races in a state where Democrats have strong recruits.

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