4 2018 Senate races that have already taken off

The 2018 midterm elections remain more than 15 months away, but the initial outlines of the battleground for control of the U.S. Senate are being sketched this summer.

With Republicans holding a spindly 52-48 majority, Democrats need to flip only three seats for a takeover. Their problem is they're carrying far more vulnerable incumbents into the cycle than Republicans are, including 10 in states carried by President Donald Trump last year.

There's no telling what the political environment will look like even six months from now, let alone by next summer. But at this early vantage point many potential GOP candidates are keeping their powder dry, assessing the long-term impact of an unpopular president under constant siege.

Emboldened by Trump's political paralysis, Democrats, meanwhile, are trying to seize the moment early in order to defy their difficult odds later.

Here's a U.S. News scouting report of four important U.S. Senate races that are already taking form in the summer of '17:

Nevada

The Race: Sen. Dean Heller (R) vs. Rep. Jacky Rosen (D)

The State of Play: Talk about a gambler. First-term Democratic Rep. Jacky Rosen declared her challenge to Sen. Dean Heller, pictured, on only her 184th day in office, a testament to her ambition as much as it is to the Republican incumbent's perceived vulnerability. Heller's facing a lose-lose situation on a potential career-altering vote to repeal and replace Obamacare. A "yes" pins him with responsibility for more than 120,000 of his residents losing health care coverage; a "no" triggers a backlash from Trump voters key to his base coalition. Rosen's reward for pulling the trigger early: A slew of endorsements from the Democratic establishment, including the party's official campaign arm, Sen. Catherine Cortez Mastro, former Senate Majority Leader Harry Reid and the abortion rights women's group EMILY's List, a fundraising powerhouse. The last two Nevada Senate races have been among the closest in the country, decided by margins of 2.4 points and including Heller's 2012 squeaker by 1.2 points. The next move belongs to Rep. Dina Titus -- a more veteran politician than the novice Rosen -- who must decide whether a primary against the Democratic machine is worth a roll of the dice.

The Early Edge: Rosen, for having the chutzpah to get in early at a precarious time for Heller. The pressure she will immediately apply on him as a challenger will help her frame the debate on her terms and solicit national cash.

Texas

The Race: Sen. Ted Cruz (R) vs. Rep. Beto O'Rourke (D)

The State of Play: It was supposed to be one of the much-hyped Castro brothers -- Julian, the former Housing & Urban Development Secretary or Joaquin, the third-term congressman -- that stepped up to take on Sen. Ted Cruz, pictured. Instead, it's a lower-wattage congressman from El Paso who is ripping up the preferred Washington playbook to try and become Texas' first Democratic senator in 24 years. Rep. Beto O'Rourke is a long way from turning The Lone Star State blue, but he's at least making Republicans sweat. There was the April survey of adults -- not likely voters -- showing him in a dead heat with the former presidential candidate. Now comes word that O'Rourke bested Cruz in second quarter fundraising, $2.1 million to the senator's $1.6 million, without taking a dime of money from political action committees. Texas shouldn't be on either party's list of top-tier races given the host of other competitive 2018 states, but O'Rourke is flirting with breakout stardom in this early round.

The Early Edge: Cruz. He still has nearly $4 million more than O'Rourke in the bank, and as he showed in the 2016 presidential race, no one runs a better technical and organization operation than his team. Plus, Democratic howls of flipping Texas have thus far proved to be more sizzle than steak.

West Virginia

The Race: Sen. Joe Manchin (D) vs. Rep. Evan Jenkins (R) and Attorney General Patrick Morrisey (R)

The State of Play: Both as governor and senator, Joe Manchin, pictured, has been a long-held prize for Republicans. But his personal charisma, centrist instincts and two decades of experience in statewide politics have been able to withstand a consistent GOP onslaught. Another test now looms with two credible elected Republicans already in the race to unseat him. Rep. Evan Jenkins is a second-term congressman who left the Democratic Party in 2013 to win his current seat as a Republican. That's a ripe contrast for Attorney General Patrick Morrisey, who is positioning himself as the authentic conservative in the primary and an unabashed loyalist to Trump, who carried West Virginia in 2016 by a mind-numbing 42 points. Morrisey is wasting no time reminding voters of Jenkins' past; the Jenkins camp is returning fire by staining Morrisey as compromised by big money interests. The best news for Manchin is that the Jenkins-Morrisey face-off is already getting ugly, creating the real potential of damaging the ultimate victor. The wildcard is if Trump decides to intervene in the primary or -- more likely -- venture into the state next fall to campaign against Manchin.

The Early Edge: Morrisey begins as the favorite. As a statewide official he's universally known and his favorability numbers among conservatives are sky-high. West Virginia is a rare state where it's a clear advantage to go full Trump. But there still could be more entries into the race given the inviting red target on Manchin's back.

Ohio

The Race: Sen. Sherrod Brown (D) vs. Treasurer Josh Mandel (R) and Businessman Mike Gibbons (R)

The State of Play: Get ready for a (likely) bitter re-run of the 2012 Ohio Senate race. State Treasurer Josh Mandel, who fell to Sen. Sherrod Brown, pictured, by 6 points in the presidential year in which President Barack Obama carried the state -- is back -- and betting the winds have shifted enough in the Buckeye State to barrel over the second-term Brown. Given that Trump defeated Hillary Clinton in Ohio by a breezy 9 points, Mandel may be right. But in order to top Brown he'll have to present himself as a more polished, scrupulous candidate. In 2012, he developed a reputation as a overanxious ladder-climber, skipping out on his day job to brazenly hobnob in Washington. It led to a toxic relationship with his home state press and tarred him as a bit of a political grifter. An alternative to Mandel has emerged in the relatively unknown banker Mike Gibbons, but Mandel's statewide name recognition and unquestionable ability to haul large sums of cash has otherwise cleared the GOP field. Even Brown admitted at the time that Obama's coattails were key to his own success in 2012. In 2018, he'll need to rally that coalition all by himself. The question is whether Trump will ultimately be a weight on Brown or Mandel.

The Early Edge: Brown. His grizzled appeal is underrated and has some crossover with Trump voters on trade and manufacturing issues. Oh, and he beat Mandel last time.

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